Why There Are Seashells on Top of Mount Everest

Pick up a piece of rock on the summit of Mount Everest and you are holding the floor of an ancient ocean. That is not a metaphor. The grey band of rock at the very top of the world — the final few metres every climber touches — is marine limestone, laid down in a warm, shallow tropical sea roughly 450 million years ago. Researchers studying samples collected just six metres below the summit have found fossil fragments of trilobites, sea lilies and tiny shelled crustaceans called ostracods.

In nearly three decades of guiding in these mountains, this is still the fact that stops people mid-stride on the trail: the highest point on Earth used to be underwater. Here is how that happened — and where ordinary trekkers can see, and even hold, this ancient ocean without climbing anything.

An ocean called Tethys

Geologists call the summit rock the Qomolangma Formation — a slab of well-bedded limestone that makes up roughly the top 125 metres of the mountain. It formed on the shallow northern continental shelf of what is now India, at the edge of a vanished tropical ocean called the Tethys. For millions of years, sea creatures lived and died in that warm water; their shells and skeletons settled into lime mud on the seafloor and were slowly compressed into stone.

Then India began to move. Drifting north across the Tethys, the Indian plate slammed into Asia around 50–55 million years ago, and the collision — which is still going on today — crumpled the old seabed like a carpet pushed against a wall. With nowhere to go but up, the ocean floor rose kilometre after kilometre until a slice of it became the summit of the highest mountain on Earth, now officially 8,848.86 metres above the sea it came from.

The climber who found the first fossils

The first person to prove any of this on the mountain itself was Noel Odell — the expedition geologist best remembered as the last man to see Mallory and Irvine alive in 1924. High on Everest that spring, at about 7,770 metres, Odell collected limestone containing the first fossils ever identified on the mountain. Later studies of the famous Yellow Band — the pale stripe of rock, roughly 172 metres thick, that rings the whole upper mountain — found it contains up to 5 per cent crinoid fragments: the crushed skeletons of sea lilies, relatives of today's starfish.

The ocean floor is still rising

The strangest part of the story is that it is not finished. A 2024 study in Nature Geoscience found that erosion by the Arun river, east of Everest, is carving so much rock out of the region that the crust beneath the mountain is slowly rebounding upward — lifting the summit by up to 2 millimetres a year. Over the past 89,000 years, researchers estimate that has already added between 15 and 50 metres of height. The old seafloor is still climbing, even as the ice around it retreats — something we covered in our report on the Khumbu Glacier's projected ice loss.

If this is the kind of fact that makes you want to stand below that summit yourself, our Everest Base Camp trekking guide covers exactly how ordinary trekkers get there — no ropes required, just two weeks of walking through the world's most improbable geology.

Source: Montana State University — Everest Education Expedition

Cover photo: Prabin Sunar via Pexels (Pexels License).