Bathymetry is the study of underwater depth & the ocean floors. Just as our maps show relief in the form of hills and mountains – topography - so we also show coastal profiles and depth contours.
Below is the first printed map of oceanic bathymetry (1836)
From a functional point of view, bathymetry is crucial to enable safe navigation across the seas, from our perspective; we’re just as fascinated with representing what’s below sea as what lies above.
Historically, undersea measurements were taken through depth sounding (from the Old English ‘sund’ meaning water, rather than ‘sound’ in terms of noise). They literally dropped a weighted rope over the side of a boat but it was notoriously unreliable due to many factors, not least the boat moving!
Below is sample of bathymetry that can be seen on our world maps.
Modern day bathymetry is mainly carried out using sonar which ‘pings’ sound down towards the sea floor - the amount of time it takes to ping back is what they use to calculate the depth. This can even be done from an airborne position or by satellite (and was what they were doing to try and locate missing plane MH370).
These pings have even been known to cause the odd sighting of Atlantis! Map Conspiracies Blog