Mount Everest stands about 8,849 metres above sea level. The deepest point in the ocean, Challenger Deep in the Mariana Trench, lies somewhere between roughly 10,935 and 10,994 metres below it. Drop the mountain into the trench and its summit would still finish more than two kilometres under the surface.

The comparison is accurate, and the gap is wide enough to absorb the uncertainty in those numbers. What is worth sitting with is the uncertainty itself.

The mountain is pinned. The trench is not.

Everest’s height is now known closely. A joint China-Nepal survey in 2020 put it at 8,848.86 metres, settling a long-running disagreement between the two countries’ official figures to within a fraction of a metre.

Challenger Deep is a different matter. A 2010 sonar survey by the University of New Hampshire, reported by NOAA, measured it at 10,994 metres with an accuracy of about 40 metres. A later analysis, published in 2021 and based on submersible dives conducted in June 2020, put the deepest observed seafloor depth at 10,935 metres, plus or minus six. Other expeditions land in between. The leading figures disagree by about sixty metres, the height of a substantial high-rise.

Two things make the deep so hard to pin down. Sound, which is how depth is measured at the bottom, does not travel through seawater at a fixed speed. It bends with changes in temperature, pressure and salinity across eleven kilometres of water, and every correction carries its own error. Challenger Deep is also not a single point but a set of basins, each more than ten kilometres down, so knowing you have found the deepest spot rather than just a deep one is a problem in itself.

Why the comparison is fair

Height-against-depth comparisons can be slippery, but this one is clean, because both numbers are measured from the same reference line: mean sea level. Everest is the highest point on Earth’s surface above it. Challenger Deep is the lowest point below it. Setting one against the other is like for like.

Everest is not the tallest mountain by every measure. Mauna Kea rises higher from base to summit once you count the part beneath the sea, and Chimborazo’s peak sits farthest from the centre of the Earth because of the planet’s equatorial bulge. For a sea-level comparison, though, Everest is the right benchmark, and it loses to the trench by a clear margin.

Why we map other worlds better than our own seafloor

The comparison has a flip side that belongs squarely on this page. We have measured the topography of Mars and Venus in finer detail than we have measured most of our own seafloor.

The reason is physical, not a lack of interest. Light and radar, the tools that map a planet’s surface from orbit, do not pass through seawater beyond the first few hundred feet, as the US Geological Survey explains. To chart the seafloor you have to bounce sound off it from a ship or a submersible, slowly, line by line. It is closer to surveying a continent on foot than to photographing it from the air.

The popular version of this, that we know more about the Moon than the ocean floor, is true in one sense and overstated in another. The whole seabed exists in global map form, but much of that picture is coarse and partly inferred from satellite measurements of the sea surface. It is good enough to show the broad shape of the ocean floor, not good enough to reveal many smaller features in detail. Topographic maps of Mars and Venus are sharper than that. But mapping in detail is a higher bar. By that standard, less than a third of the ocean floor has been mapped at high resolution: NOAA puts the figure at 28.7 per cent as of April 2026, up from only six per cent when Seabed 2030 began.

So the deepest place on Earth is, in the way that matters for precision, less well charted than craters you can pick out on the Moon through a backyard telescope.

What to keep from the factoid

The comparison holds.

Everest really would sit more than two kilometres beneath the waves at the bottom of the Mariana Trench, and the margin is wide enough that the uncertainty in the trench’s depth does not threaten it.

The sharper point is the one underneath. We can state the height of the mountain to within a metre and the depth of the trench only to within tens of metres, and that gap says something about how much harder it is to see through water than through vacuum.