Enigma 2 solution : the seefloor, how you know how deep is it ?

Yes, we had to find 5 250 m depth for the Hess Deep Rift ...  option 3 !

The reasons are simple: the acoustic waves (those used by scientists) are faster in water than in air. The speed of these waves in the sea water is about 1 500 m / s (only 300 m / s in the air!).

Then 7 seconds to go & back, that's for 3.5 sec. each way then 3.5 x 1 500 = 5 250 m.

Bathymetry rift
By multiplying the points of measurement, we can map the depth of the ocean floor: 'bathymetry'. The Hess Deep Rift is aptly named !

More about depth of the see floor below :