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.
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 :