At more than 700 metres deep in the Atlantic Ocean, near the top of an underwater mountain west of the Mid-Atlantic Ridge, stands something scientists call the "Lost City" - a field of pale limestone towers that may hold one of the biggest answers: how life began at all.

Discovered in 2000, this hydrothermal field has been active for at least 120,000 years - the longest-known such environment in any ocean. The tallest tower, named Poseidon after the Greek god of the sea, rises 60 metres. Unlike the volcanic "black smokers" that depend on the heat of magma, the Lost City produces up to a hundred times more hydrogen and methane - and does so through purely chemical reactions between rocks from the Earth's mantle and seawater, with no sunlight and no atmospheric carbon dioxide. And those very gases are considered the building blocks of life.

And despite the extreme conditions, life flourishes there - microbial communities without oxygen, dense populations of snails and crabs, and at times larger creatures too. "This is an example of an ecosystem that could be active right now on Enceladus or Europa," says microbiologist William Brazelton, referring to the icy moons of Saturn and Jupiter. In other words, if life could arise here, it could there too.

But a shadow already hangs over this rarity. In 2018 Poland was granted deep-sea mining rights nearby, which stirs scientists' fear that clouds of sediment could smother the fragile ecosystem before we even understand it. And here is the eternal human story in miniature: we find a place that may hold the secret of our origins - and the first thing we think of is how to dig something out of it. Scientists are seeking protection under UNESCO. The question is whether understanding will arrive before the diggers.