How are we sure Challenger Deep is the deepest point when we have barely fully explored our ocean?

Question

Basically the title, but I always wondered if the Mariana Trench was the actual deepest point on our planet

In: 56

Answer 1

A boat on the surface of the ocean can use sonar to measure the depth of the ocean. We may not have been to the bottom at every location but our sonar pings have. In doing so, we have mapped the depth across the ocean floor to reasonable detail, enough to know there is nothing deeper.