Everest Is the Highest Mountain, but Not the Tallest

Picture a beach on the Big Island of Hawaii: warm sand, a row of palm trees, and behind them a broad, gentle mountain with a dusting of snow on its top. It does not look like a record breaker. Yet by one perfectly fair way of measuring, that quiet giant, Mauna Kea, is taller than Mount Everest. And a third mountain, far away in Ecuador, beats them both at something else.

Ask anyone which mountain is the biggest on Earth and the answer comes back at once: Everest. It is a good answer. It is just the answer to one particular question, and there are at least three questions hiding in that word biggest.

What people often get wrong

The mistake is to treat highest and tallest as the same thing. Everest's summit is the highest point above sea level, at 8,848 metres, according to the US National Oceanic and Atmospheric Administration (NOAA). Start counting at the surface of the sea and nothing on the planet reaches further up. But sea level is a starting line that people chose. A mountain does not know where the water is.

Mauna Kea begins on the floor of the Pacific Ocean. NOAA says it rises more than 10,210 metres from base to peak, and Guinness World Records, which lists it as the world's tallest mountain, gives about 10,205 metres. The two figures differ by a few metres, and either way it comfortably beats Everest. Only 4,205 of those metres are above the waves, which is less than half of Everest's height. The rest is underwater, where nobody takes holiday photographs.

• 8,848 m Everest: the highest summit above sea level
• 10,205 m Mauna Kea from base to peak, with only 4,205 m above the sea
• 2,072 m how much farther Chimborazo's summit is from Earth's centre than Everest's

Why the shape of the Earth changes the answer

Then there is Chimborazo, a snowy volcano in Ecuador. Its summit is 6,268 metres above sea level, more than two and a half kilometres lower than Everest. But the Earth is not a perfect ball. NOAA explains that the planet's constant spinning makes it a bit thicker around the Equator, and Chimborazo sits just one degree south of the Equator, where that bulge is greatest. Measure from the centre of the Earth instead of from the sea, and Chimborazo's summit is more than 2,072 metres farther out than Everest's. Stand on top of it and you are as far from the middle of the planet as you can get with your boots still on the ground.

How we know

None of this needs a new expedition, only a different ruler. For Mauna Kea, the sums are simple. Guinness World Records notes that the sea floor beside the island, in a deep called the Hawaiian Trough, lies some 5,500 to 6,000 metres down. Add the deepest part of that to the 4,205 metres that show above the water, and you arrive at the record. For Chimborazo, the key is the bulge: the ground near the Equator is already farther from the centre of the Earth before any mountain starts to rise.

So which mountain is the biggest? All three, depending on what you ask. Everest is the highest above the sea, Mauna Kea is the tallest from foot to top, and Chimborazo is the farthest from the centre of the Earth. The next time the question comes up at a quiz night, the correct reply is a polite one: measured from where?