Underwater exploration is the exploration of any underwater environment, either by direct observation by the explorer, or by remote observation and measurement under the direction of the investigators. Systematic, targeted exploration is the most effective method to increase understanding of the ocean and other underwater regions, so they can be effectively managed, conserved, regulated, and their resources discovered, accessed, and used. Less than 10% of the ocean has been mapped in any detail, less has been visually observed, and the total diversity of life and distribution of populations is similarly obscure.
Types of exploration include investigation of the form and extent of the body of water or part thereof, investigation of the geological characteristics of the seabed and freshwater equivalents, and investigation of the geological structure, strata, and sediments underlying the body of water, investigation of the physical and ecological characteristics of the body of water and its containing geographical features, discovery and investigation of shipwrecks and archeological sites, and direct and remote visual observation of what is there.
The oceans can be divided into deep ocean and coastal waters. Inland waters are mostly fresh, and consist of rivers, lakes and ground water, some of which is in accessible caves.
Underwater exploration is largely a recent development, as it relies heavily on fairly advanced technology over almost all of the relevant territory.
Exploration is the process of exploring, which has been defined as (amongst other possible meanings):
According to the definition. this does not necessarily require the explorer to be present at the point of investigation, so exploration of the underwater environment by remotely operated equipment, remote measurement, and autonomous devices programmed to explore the underwater environment is included. Surveys of the underwater environment, particularly geographical surveys, are also considered to be underwater exploration in a broad sense, as they are a form of systematic investigation for information not yet known in the desired detail.
Systematic investigation is the realm of science, both professional and amateur. Particularly when findings are published in some way for the benefit of the wider community. Some results of systematic exploration are kept hidden from the general public for commercial and political reasons.
To travel in search of discovery is a luxury that most people and organisations cannot afford. This form of exploration is largely the domain of the wealthy and the dedicated. When financial backing occurs, the investigations tend to become systematic and targeted.
To seek experience first hand and to wander without any particular aim or purpose is recreation, and the discoveries from this aspect of exploration are likely to be personal, limited, and repetitive, as various people explore the same environment at different times. Discovery by this route tends to be published only when something obviously unusual is found, and is likely to be published as news.
The scope of underwater exploration includes the distribution and variety of marine and aquatic life, measurement of the geographical distribution of the chemical and physical properties, including movement of the water, and the geophysical, geological and topographical features of the Earth's crust where it is covered by water.
Systematic, targeted exploration is the most effective method to increase understanding of the ocean and other underwater regions, so they can be effectively managed, conserved, regulated, and their resources discovered, accessed, and used. The ocean covers approximately 70% of Earth’s surface and has a critical role in supporting life on the planet but knowledge and understanding of the ocean remains limited due to difficulty and cost of access.
The distinction between exploration, survey, and other research is somewhat blurred, and one way of looking at it is to consider the baseline surveys and research as exploration, as previously unknown information is gathered. Updating and refining the data is less exploratory in nature, but may still be exploration for the people involved, in the sense that the experience is new to them.
According to NOAA, as of January 2023: "More than eighty percent of our ocean is unmapped, unobserved, and unexplored." Less than 10% of the ocean, including about 35% of the ocean and coastal waters of the United States, have been mapped in any detail using sonar technology.  According to GEBCO 2019 data, less than 18% of the deep ocean bed has been mapped using direct measurement and about 50% of coastal waters were not yet surveyed.
Most of the data used to create seabed maps are approximate depths derived from satellite gravity measurements and sea surface heights which are affected by the shape and mass distribution of the seabed. This method of approximation only provides low resolution information on large topographical features, and can miss significant features.
The extent of flooded caves is largely known as a result of underwater exploration of those caves.
The oceans can be divided into deep ocean and coastal waters. Inland waters are mostly fresh, and consist of rivers, lakes and ground water, some of which is in accessible caves.
Main article: Deep-sea exploration
Deep-sea exploration is the investigation of physical, chemical, and biological conditions on the sea bed, and water column beyond the continental shelf for scientific, commercial or other purposes. Deep-sea exploration is considered a relatively recent human activity compared to the other areas of geophysical research, as the depths of the sea have been investigated only during comparatively recent years. The ocean depths still remain a largely unexplored part of the Earth, and form a relatively undiscovered domain.
In general, modern scientific deep-sea exploration can be said to have begun when French scientist Pierre-Simon de Laplace investigated the average depth of the Atlantic ocean by observing tidal motions registered on Brazilian and African coasts. He calculated the depth to be 3,962 metres (12,999 ft), a value later proven quite accurate by echo-sounding measurement techniques. Later on, due to increasing demand for the installment of submarine cables, accurate measurements of the sea floor depth were required and the first investigations of the sea bottom were undertaken. The first deep-sea life forms were discovered in 1864 when Norwegian Michael Sars obtained a sample of a stalked crinoid at a depth of 3,109 m (10,200 ft).
Coastal waters and waters of the continental shelf have been explored more systematically than deeper waters, since they are to a large extent within the exclusive economic zone of adjoining countries, and are relatively accessible.
Inland waters. Mostly, but not exclusively fresh water.
The exploration of lakes is done in much the same way as exploration of coastal waters, and is often simplified by relatively small water movement, and shallow depths, though depths can easily extend beyond the range for ambient pressure diving. Visibility ranges between the best, nearing the theoretical maximum for water, and the worst, effectively zero, or measurable in millimeters.
Rivers have the complication of flow, which can range from sluggish to rapid and extremely turbulent, but are relatively shallow.
Many lakes and rivers are relatively easily accessible, others are in inaccessible places.
The underwater environment in flooded caves is a relatively difficult and dangerous environment for exploration. There is a lack of natural light, limited line of sight, a general lack of free surface, and often very restricted space. The water can also be quite deep, and there may be strong flow of the water. Cave-diving is underwater diving in fresh or seawater-filled caves. It may be done as an extreme sport, a way of exploring flooded caves for scientific investigation, or for the search for and recovery of other cave users. The equipment used varies depending on the circumstances, and ranges from breath hold to surface supplied, but almost all cave-diving is done using scuba equipment which gives the diver greater range and autonomy, but with a limited breathing gas supply, often in specialised configurations with redundancies such as sidemount or backmounted twinset. Remotely operated vehicles and autonomous underwater vehicles are also used for the exploration of flooded caves, as they do not risk human life and have a far greater operating depth range.
Underwater cave mapping is complicated by both a lack of access to the surface for GPS positions, darkness, with short line-of-sight, and limited visibility, which complicate optical measurement. Altitude/depth is relatively simple as accurate depth measurement is available to divers in the form of decompression computers, which log a depth/time record of reasonable accuracy and are available for instantaneous readout at any point, and depth can be referenced to the altitude at the surface. Vertical dimensions can be directly measured or calculated as differences in depth.
Surface coordinates can be collected via GPS and remote sensing, with varying degrees of precision and accuracy depending on the type of entrance. In some caves the water surface is in view of GPS satellites, in others it is a considerable distance along a complex route from the nearest open air. Three dimensional models of varying accuracy and detail can be created by processing measurements collected by whatever methods were available. These can be used in virtual reality models. The usual methods for survey and mapping of underwater caves are dead reckoning and direct measurements of distance, compass direction and depth, by diving teams of two or three scuba divers, who record azimuth of the cave line, measurements of height, width, depth, and slope at intervals along the line, generally using a permanent guide line as a reference baseline, and take photographic records of features and objects of interest. Data are collected onand by digital photography. Where the depth or other constraints prevent divers from exploring in person, tethered and untethered remotely operated underwater vehicles (ROUVs) have been used effectively, using sonar technology to scan and map the surroundings, and video to record the appearance.
Features, artifacts, remains, and other objects of interest are recorded in situ as effectively as possible, generally by photography.
Unknown or poorly known parts of the underwater environment may be explored directly by human observers, or measured and recorded by instruments. Direct measurements and remote measurements are used to suit circumstances. Direct observation is often used investigating new territory, and it is not yet known what to expect, or what instrumentation may be most useful, while remote measurements tend to be faster and where possible, produce useful results sooner and at lower cost, but are more limited in what they can observe.
Direct measurements and observations of underwater objects and water properties may be done from a surface platform, by instruments deployed from a surface platform, by divers, from crewed submersibles, ROUVs, or AUVs. Direct observation is mostly used when remote observation is impracticable, impossible, or when accuracy requirements dictate. Most water properties other than velocity and surface temperature, such as temperature at depth, salinity, density, transparency, solute composition and particulate load require direct measurement, which may be done in situ or by recovering samples and testing them in the laboratory.
Surface platforms are commonly used as a base on which to mount, or from which to deploy instrumentation, which may be immediately recovered, or left to record data and recovered later.
See also: Underwater diving § Diving activities
Divers are limited in mobility and range, but can interact directly with the environment which gives them great flexibility and precision of interaction, but are not inherently efficient for precise measurement of the broader environment, for which they need tools and recording equipment.
Main article: Submersible
Crewed submersibles have a much larger depth and lateral range than divers, but are less dexterous at precision manipulation and handling delicate materials and organisms. The presence of an operator with a direct view of the environment makes them logistically flexible, and plans can be changed on the fly to take advantage of serendipitous discoveries. Crewed submersibles allow personal exploration of otherwise inaccessible ocean depths, and can perform a variety of observation, sampling and measurement tasks.
An autonomous underwater vehicle (AUV) is a robot that travels underwater without requiring continuous input from an operator. AUVs constitute part of a larger group of undersea systems known as unmanned underwater vehicles, a classification that includes non-autonomous remotely operated underwater vehicles (ROVs) – controlled and powered from the surface by an operator/pilot via an umbilical or using remote control. Underwater gliders are a subclass of AUVs.
A remotely operated underwater vehicle (technically ROUV or just ROV) is a (usually) tethered underwater mobile device, that is unoccupied, usually highly maneuverable, and operated by a crew, either aboard a support vessel, floating platform or on proximate land. They are generally, but not necessarily, linked to a host ship by a neutrally buoyant tether or, often when working in rough conditions or in deeper water, a load-carrying umbilical cable is used along with a tether management system (TMS). The TMS is either a garage-like device which contains the ROV during lowering through the splash zone or, on larger work-class ROVs, a separate assembly mounted on top of the ROV. The purpose of the TMS is to lengthen and shorten the tether so the effect of cable drag where there are underwater currents is minimized. The umbilical cable is an armored cable that contains a group of electrical conductors and fiber optics that carry electric power, video, and data signals between the operator and the TMS. Where used, the TMS then relays the signals and power for the ROV down the tether cable. Once at the ROV, the electric power is distributed between the components of the ROV. In high-power applications, most of the electric power drives a high-power electric motor which drives a hydraulic pump. The hydraulic system is then used for propulsion and to power equipment such as torque tools and manipulator arms where electric motors would be too difficult to implement underwater. Most ROVs are equipped with at least a video camera and lights. Additional equipment is commonly added to expand the vehicle's capabilities. These may include sonars, magnetometers, a still camera, a manipulator or cutting arm, water samplers, and instruments that measure water clarity, water temperature, water density, sound velocity, light penetration, and temperature.
See also: Remote sensing
Remote sensing is usually more efficient for tasks covering large areas, such as bottom surface profiling, reflection seismology or measuring sea surface temperature over wide areas, and is generally used where the technology is available. Remote sensing is usually automated to some extent, and signal processing and data storage and analysis may also be automated. The remote sensing platform is often a surface vessel, but may also be a crewed submersible, ROV, AUV, aircraft of satellite. On occasion remote sensors have been carried by divers.
Water depth measurable by lidar depends on the clarity of the water and the absorption of the wavelength used. Water is most transparent to green and blue light, so these will penetrate deepest in clean water. Blue-green light of 532 nm produced by frequency doubled solid-state IR laser output is the standard for airborne bathymetry. This light can penetrate water but pulse strength attenuates exponentially with distance traveled through the water. The surface reflection makes water shallower than about 0.9 m difficult to resolve, and absorption limits the maximum depth. Turbidity causes scattering and has a signifincant role in determining the maximum depth that can be resolved in most situations, and dissolved pigments can increase absorption depending on wavelength. Bathymetric lidar is most useful in the 0 to 10 m depth range in coastal mapping. On average in fairly clear coastal seawater lidar can penetrate to about 7 m, and in turbid water up to about 3 m. An average value is for the green laser light to penetrate water about 1.5 to 2 times Secchi depth. Water temperature and salinity have an effect on the refractive index which has a small effect on the depth calculation.
The ICESat-2 satellite has a laser altimeter intended for measuring the height of ice, but it was found that underwater reflections were also being recorded along shallow coastal zones. This has allowed areas where it is too shallow for most vessels to safely access to be bathymetrically mapped. The potential depth that ICESat-2’s Advanced Topographic Laser Altimeter System (ATLASA) can reach is 38 m in optimum conditions.
A magnetometer is an instrument that measures magnetic field or magnetic dipole moment. They are widely used for measuring the Earth's magnetic field, in geophysical surveys, to detect magnetic anomalies of various types, and to find iron and steel shipwrecks.
Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest.
Sonar is the most effective technology for underwater surveying, as sound propagates through water with less loss than electromagnetic energy, reflects well at a phase interface, can be produced at a wide range of frequencies with varying applications, and can be directionally focused with some precision, but the transducers must be in the water – sound does not propagate well through a gas-liquid or liquid-solid interface. Three main applications are used: single beam echo sounding is generally used for depth measurement below the vessel, side-scan sonar produces images showing the shape of underwater objects well, but is not very accurate for depth measurement, and multibeam echosounders provide fairly accurate three dimensional positions for a swath of points spread across the track of the transducer array.
Underwater photography and Underwater videography
Undersea exploration is usually conducted from a platform, such as a ship, buoy, aircraft, or satellite. A seaborne surface platform may be used as a base to deploy divers, crewed submersibles ROUVs and AUVs, or may be directly equipped with remote sensors. Aircraft and satellites may carry remote senors with a longer range and wider view.
Scientists estimate that the ocean contains between 700,000 and 1 million living species (excluding most microorganisms, of which there are estimated to be millions more), most of which are yet to be found and described.
See also: Cave diving § History
The 1872–76 Challenger expedition was the first major multidisciplinary undersea survey, which had the primary goal of discovering deep-sea life using dredging and nets, and also made physical, oceanographic, and chemical measurements of the oceanic environment.
Work on mapping the ocean bed accelerated after world war 2, when sonar technology made faster depth measurement possible.
The first comprehensive map of the world ocean bottom was published in 1977 by geologists Marie Tharp and Bruce Heezen of Lamont Geological Laboratory at Columbia University in New York, in a collaboration that lasted from the 1950s into the 1970s. In the 1980s, William Haxby used satellite measurements to provide more information, using gravity field data to provide a low resolution map of the global seafloor.
The mass distribution of the seabed topography affects the local gravity sufficiently for satellite radar altimetry to record variations of sea surface height, which can be used to calculate the approximate underwater geomorphology. The map created in 2014 using this data revealed large numbers of previously unknown seamounts, and has roughly twice the resolution of the previous map created 20 years earlier. Data was used from the European Space Agency’s (ESA) CryoSat-2 satellite and NASA’s Jason-1 satellite.
The First World Ocean Assessment of 2015 showed that the ocean is important for the climate and support of life on the whole planet, and that critical ocean systems are under threat, making the exploration and mapping of the ocean a key environmental goal to facilitate understanding of the dynamics of ocean systems and the changes that are occurring.
By 2017 only about 6% of the ocean floor had been mapped. The United Nations Ocean Conference of that year challenged nations to complete that map and by 2020 the coverage had increased to about 20%.
In June 2022, the US formally joined the Seabed 2030 project, along with a number of other countries. At that time 23.4% of the seabed had been mapped, which included an increase of 10.1 million square kilometers of new bathymetric data from 2022, contributed by a diverse group of participants, including national governments, private companies, academic institutions and philanthropic partners.