Everest is the highest mountain but not the tallest, and both are true

Treat the sections below as a sequence. With which mountain is the biggest, getting the early decisions right makes the later ones much easier.

The measurement everyone quotes

The famous figure for Everest is a height above mean sea level, which is a reasonable convention and also a slightly strange one for a mountain nowhere near a sea. It was first established by a nineteenth-century survey working across the plains of the subcontinent using trigonometry and extremely long sight lines.

That survey had to model the shape of the Earth and the gravitational pull of the mountains themselves on its own instruments, which is harder than it sounds. Modern figures come from satellite positioning combined with a model of the gravity field, and they have been revised more than once since. A joint measurement announced by the two countries the mountain sits between adjusted the accepted number again in recent years.

Measuring from the base instead

If you ask how much mountain there is, rather than how far above the sea the top of it sits, the answer changes completely. A Hawaiian volcano rises from the ocean floor and continues well above the surface, giving a base-to-summit figure considerably larger than Everest's quoted height.

Most of it is underwater, which is why nobody mentions it, and its slope is so gradual that standing on it does not feel like a mountain at all. The base of a continental mountain is genuinely hard to define, because the plateau it stands on is itself several kilometres above the sea. Everest's summit is roughly four kilometres above the surrounding high country, which is impressive but is not the number in the encyclopaedia.

Measuring from the centre of the Earth

The planet is not a sphere but is flattened at the poles and bulges at the equator by roughly twenty kilometres of radius. A mountain sitting almost exactly on the equator therefore starts from a radius already much further out than one in the northern subtropics.

By that measure a volcanic peak in Ecuador has its summit further from the centre of the Earth than any other point on the surface. This is the right measurement if you care about distance from the planet's core, and it is the wrong one for absolutely everything else. It is also the answer that most reliably starts an argument, because it feels like cheating while being straightforwardly correct.

Sea level is itself a model

There is no single sea level, because the ocean surface is lumpy, following variations in the gravity field caused by uneven density inside the Earth. Surveyors use a modelled surface that the sea would settle into if it could flow freely beneath the continents, and different countries have historically used different versions. The differences are metres rather than kilometres, but they are quite large enough to matter when two nations are comparing figures for the same summit.

Snow depth adds another layer of disagreement, since the rock summit and the snow summit are not at the same height and the snow moves.

Announced heights therefore sometimes specify whether the snow cap is included, a level of precision that most news reports quietly drop.

The mountain is also moving

The range is being pushed upward by the collision of two continental plates, and satellite measurements show a slow but continuing rise. Erosion removes material at the same time, so the net change is a small difference between two much larger opposing processes. Earthquakes can shift a summit horizontally and vertically in a single event, which is one reason measurements are repeated after a large one.

Any figure quoted to the centimetre is a snapshot with a date attached rather than a permanent property of the mountain itself. That is unusual among famous numbers, most of which describe things that at least have the courtesy to stay still.

Where the earliest trace is a newspaper anecdote, treat the whole chain with suspicion.

Why this is not a trick question

Each answer is correct under a stated definition, and the whole disagreement is about which question somebody is asking. Highest above sea level is the most useful measure for climbing, weather and the thinness of the air, which is why it became the standard one. Base to summit is the right measure for the geology of how a mountain was built, and distance from the centre is right for the shape of the planet.

Tested properly, debunkings announcing that Everest is not really the biggest are overselling a definitional point that geographers have never attempted to hide. The interesting content is that a simple superlative turns out to need a specified starting line, which is true of a surprising number of records.

The takeaway

Every superlative needs a starting line. Change the starting line and three different mountains win.

Ask who gained from the claim being repeated. That usually explains the rest.