In the 45°C heat of the midday April sun, I swing my sledgehammer into the terracotta-varnished lobes of pillow basalt overlooking a sparse, almost Martian landscape.

Up close, the rock is freckled with small spheres or varioles, a texture that forms in wet magmas. It’s hard to fathom that this lava cooled when Earth was young, and has barely changed since.

Western Australia’s Pilbara Craton is probably the last place you’d expect to learn anything about the role water played in shaping our planet. It’s one of the hottest places on Earth. The land is dry and largely barren, save for the sharp spines of spinifex grass and the occasional gum tree or acacia.

Yet our research, published today in Nature Communications, shows that a rare package of unusually well-preserved rocks from this area documents the movement of surface water to Earth’s interior more than 3.1 billion years ago.

A second, slower water cycle

We learn the water cycle in school as a story of evaporation and precipitation. But Earth runs a second, far slower one deep beneath our feet.

On the ocean floor, seawater seeps into the oceanic crust and reacts with the rock, becoming chemically bound inside its minerals, locked into their crystal structures rather than sitting as free........

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