The video, posted with the unhelpful caption “why were all the mountains under the ocean” has led to questions and claims that it is due to the “great flood” of the Bible.

Of course, fossils up a mountain are not evidence of a great flood. But when they were first found, they took a little explaining. An early idea came from Leonardo da Vinci, who had found fossils in a mountain cave, and was presented with marine fossils by locals when he was living in Milan. Da Vinci recognized that it must be due to the changing landscape of Earth, and that the mountains must have been at sea-level when the marine life met its end. He believed that pieces of Earth’s crust may have fallen into cavities filled with water, which in turn pushed up other nearby pieces of crust.

Though not correct, it was a little closer to the truth than “fish swam there during the flood”. Unfortunately, he did not publish these ideas, and for centuries many naturalists pushed the fossils as evidence in favor of the flood. 

Later ideas included the idea that the Earth contracts as it cools, forming basins and mountains as it did so. But this doesn’t make much sense either; if that were the case you would expect mountains to be roughly evenly distributed across the planet’s landmasses, rather than in the long chains we actually have on Earth.

The real answer came much later when we finally learned about plate tectonics in the 1960s, a development from the continental drift theory first proposed in the 1920s.

At the summit of Mount Everest, for example, there is the sedimentary rock limestone known as the “Qomolangma Limestone“. Within it are several fossilized marine creatures from the Ordovician Period, 488.3 million-443.7 million years ago. Such fossils are found across the Himalayas, and finds include trilobites, brachiopods, ostracods and crinoids.

Almost all sedimentary rocks are formed by water erosion, grinding up rocks over thousands or millions of years, before they are compacted and turned under pressure into sedimentary rock. The sedimentary rock and the presence of ancient marine creatures tell us that the rock at the summit of Mount Everest was once underwater. It also tells us that something happened to bring that rock over 8,000 meters (26,200 feet) above sea level.

Everest and the Himalayas were formed in a collision between the Eurasian and Indian continental plates, which began around 40-50 million years ago.

“The Eurasian plate was partly crumpled and buckled up above the Indian plate but due to their low density/high buoyancy, neither continental plate could be subducted,” The Geological Society explains. “This caused the continental crust to thicken due to folding and faulting by compressional forces pushing up the Himalayas and the Tibetan Plateau.”

And that’s why there are marine fossils on the (not quite) highest mountain in the world, and many other mountains on Earth.