Sonar is still the most efficient way to map the seabed. However, it is worth noting that only an insignificant part of the underwater "land" has been mapped so far, although it accounts for up to 70% of the Earth's surface.
The reason is the inability to scan the seabed from the surface (similar to how land is mapped using radars and lidars), since sound waves literally "dissolve" in water, losing up to 99.9% of energy when passing from one medium to another.
The capabilities of shipborne sonars located in the underwater part of ships are usually limited to the search for submarines and other specific objects. A team of scientists from Stanford proposed an original solution to this problem - to use, instead of devices that generate electromagnetic radiation and sound waves, a "surface" pulsed laser that generates sound waves under the surface of the water.
The PASS photoacoustic aviation system created on the basis of this idea turns on the laser above the water surface, and it takes some time for it to select the required radiation frequency. As the energy of the laser beam is absorbed in the water, sonar-like ultrasonic waves are generated that can bounce off the bottom and return to the surface.
When passing from water to atmosphere, sound waves also lose most of their energy. However, what is left is quite enough to generate a signal that can be "heard" by transducers on board an aircraft or drone with the PASS installed.