Sonar
Definition
Seeing with sound underwater — how submarines, dolphins, and fish-finders navigate the dark ocean.
Technical definition
SOund Navigation And Ranging: active (ping and listen) or passive (listen only). Sound travels about 1500 m/s in seawater.
Example
Dolphins evolved sonar millions of years before humans — their clicks can distinguish a fish from a rock by echo texture alone.
Common confusion
Sonar can't map the whole ocean at once — absorption and noise limit its range. That is why most of the seafloor remains unmapped.