How deep is the Mariana Trench, in feet?

How to get close to this number

The reliable anchor for ocean depth is a mountain. If you know roughly how tall the highest peak on land is, you already have the right order of magnitude for the deepest point in the sea — and the deep wins, with room left over. Estimating downward from a mountain you can picture beats trying to imagine a vertical column of water, which nobody has any intuition for.

What makes this one hard is that the ocean does not look deep. The average depth of the sea is a small fraction of the trench, and photographs of the surface offer no cue at all, so people reach for a number that feels large in everyday terms — a few thousand feet — and land far short. Trenches are not the ocean floor being deep; they are a place where one tectonic plate is being driven underneath another, which is a different mechanism and a different scale.

There is genuine measurement uncertainty in the last few metres, and published figures for the Challenger Deep differ slightly depending on the survey, the sound-speed model used to convert sonar timings into distance, and even the tide at the moment of the reading. That spread is small against the depth itself but it is why this figure is quoted as approximate and stamped with a year rather than presented as settled.