Thr caption to the lead image says "captured by satellite." For those who are curious, it is the slight variations in gravity strength that the satellite orbit reacts to which, when processed through an geophysical algorithm, give a reasonably good approximation for ocean depth at the location beneath the satellite. Actual bathymetric measurements confirm the general accuracy of this approach, but it can miss some important small features. "Small" in comparison to the height of the satellite above the Earth.