The seafloor has been mapped using a progression of technologies, from weighted lines dropped overboard to satellite radar that measures ocean surface height from space. Today, the primary tool is sonar, which bounces sound waves off the bottom to measure depth and reveal terrain. Despite centuries of effort and modern advances, only 27.3% of the ocean floor has been mapped to modern standards as of 2025.
Lead Lines: The Earliest Method
Until the 1920s, the only way to measure ocean depth was the lead and line method. A lead weight on a rope was dropped from a ship’s deck and lowered until it hit bottom. The length of rope paid out gave the depth. This technique worked, but it was painfully slow. Each measurement captured a single point, and a ship had to stop or drift while the weight sank. Mapping anything beyond a harbor or shipping lane in meaningful detail was essentially impossible.
Echo Sounding and Single-Beam Sonar
Echo sounding devices, first developed in 1915, gradually replaced lead lines. The concept is straightforward: a device on the ship’s hull sends a pulse of sound downward, and the time it takes for the echo to return reveals the depth. A single-beam echosounder captures one depth reading per ping, tracing a narrow line of the seafloor along the ship’s track. This was the technology behind the first large-scale ocean floor surveys, including the work that revealed mid-ocean ridges and helped confirm plate tectonics in the mid-20th century.
Multibeam Sonar
Multibeam sonar is the workhorse of modern seafloor mapping. Instead of sending one beam straight down, a multibeam system uses an array of transducers mounted on the ship’s hull, each pointing at a slightly different angle. The result is a fan-shaped swath of sound pulses that can generate up to 864 separate depth measurements in a single ping. The width of the swath on the seafloor is typically more than six times the water depth, so in 1,000 meters of water, one pass of the ship maps a strip over 6 kilometers wide.
These systems produce detailed three-dimensional maps in depths ranging from 10 to 7,000 meters or more. But there’s an inherent trade-off: the deeper the water, the lower the resolution. Sound pulses spread as they travel, so by the time they reach an abyssal plain 4,000 meters down, each ping covers a much larger patch of seafloor than it would in shallower water. Fine details get smoothed out.
Running a research vessel equipped with multibeam sonar costs roughly $30,000 per day, and a ship can only survey so much ground. That cost and pace help explain why nearly three-quarters of the ocean floor remains unmapped at high resolution.
Side-Scan Sonar
Where multibeam sonar measures depth, side-scan sonar creates something closer to a photograph. It sends sound pulses out to the sides and records the strength of the returning echoes. Hard objects like rocks, shipwrecks, or coral send back strong signals and appear dark in the image. Soft materials like mud and sand return weaker echoes and appear light. Shadows behind protruding objects add a sense of three-dimensionality, much like shadows in an aerial photo taken at a low sun angle.
Side-scan sonar cannot measure depth on its own, so it’s typically used alongside multibeam systems to build a more complete picture. It’s widely used for mapping shipwrecks and cultural heritage sites, characterizing bottom composition, and identifying marine habitats.
Synthetic Aperture Sonar
Synthetic aperture sonar, or SAS, is an advanced form of side-scan that dramatically improves image quality. Traditional side-scan loses resolution the farther the sound travels from the transducer. SAS solves this by overlapping its pulses so that each patch of seafloor is measured multiple times as the vehicle moves forward. Software then combines those overlapping returns to artificially extend the effective length of the sensor array. The result is imagery at roughly 30 times the resolution of traditional side-scan sonar, with consistent clarity across the entire swath. Archaeological surveys commonly use SAS because it can resolve fine structural details on the seafloor.
Underwater Vehicles for Deep, High-Resolution Work
To map the deepest parts of the ocean in sharp detail, the sonar needs to be closer to the bottom. Two types of underwater vehicles make this possible. Remotely operated vehicles (ROVs) are tethered to a ship and controlled by a pilot on the surface. Autonomous underwater vehicles (AUVs) are pre-programmed robots that navigate independently. Both can carry multibeam sonar systems and fly tens of meters above the seafloor, capturing features that a surface ship thousands of meters overhead would miss entirely.
Ship-mounted sonar looking down from the surface tends to smooth out rugged terrain and completely misses overhanging cliffs or steep vertical walls. AUVs and ROVs get close enough to resolve these structures. Some are also equipped with stereo cameras or photogrammetry systems that build three-dimensional models from overlapping images. On coral reefs, this technique can reconstruct individual coral colonies in enough detail to see single polyps. Deep-sea camera systems mounted on AUVs can image the seafloor from up to 9 meters altitude, allowing rapid coverage of complex habitats.
Satellite Altimetry
Satellites can’t see the seafloor directly through kilometers of water, but they can infer its shape by measuring the ocean surface with radar. Large underwater features like seamounts and ridges have extra mass, which creates a slightly stronger gravitational pull. That pull draws water toward the feature, producing a subtle bump in sea surface height, sometimes just a few centimeters. Satellite altimeters detect these bumps with extreme precision, and scientists combine the gravity data to build global maps of seafloor topography.
The resolution is far coarser than sonar. Previous ocean-observing satellites could only detect seamounts taller than roughly 1,000 meters. NASA’s SWOT satellite, launched in late 2022, can pick up seamounts less than half that height, potentially increasing the number of known seamounts from 44,000 to 100,000. Still, satellite-derived maps show broad shapes, not fine detail. A seamount that rises just 600 meters from its base can register in the gravity signal, but its surface texture, composition, and precise contours require sonar to resolve.
Satellite altimetry’s great advantage is coverage. It can survey the entire ocean, including remote areas no ship has visited. It serves as a reconnaissance tool, identifying where to send ships for detailed sonar surveys.
Airborne Lidar for Shallow Water
In coastal and shallow water, airborne lidar fills a gap that sonar ships handle awkwardly. An aircraft or drone carries a laser scanner that fires two wavelengths simultaneously. An infrared pulse reflects off the water surface, while a green laser pulse (at 532 nanometers) penetrates the water and bounces off the bottom. The time difference between the two returns gives the water depth.
This approach maps both the land and the underwater terrain in a single flight, which is especially useful for coastlines, river mouths, and coral reefs. A survey off the coast in 2021 using this technology collected about 60.8 million data points across roughly 1,152 hectares, with a ground density of 6 to 8 points per square meter and a maximum water penetration of 17.5 meters. In murkier water, penetration drops significantly, from about 7 meters in clear conditions to just 3 meters in turbid water. Lidar bathymetry is used for flood prediction, shoreline erosion monitoring, navigation charting, and managing offshore resources.
How Much Remains Unmapped
The Nippon Foundation-GEBCO Seabed 2030 Project, an international effort to map the entire ocean floor, announced in June 2025 that 27.3% of the world’s seafloor has now been mapped to modern standards. That leaves nearly three-quarters of the ocean bottom known only through low-resolution satellite gravity data or not surveyed at all. The challenge is straightforward: ships are expensive and slow, the ocean is vast, and sonar resolution degrades with depth. Closing the gap will require coordinated use of every technology available, from satellite reconnaissance to AUV swarms operating close to the bottom.