It’s hard to imagine that under this small metal cap lies the world’s deepest borehole. Now surrounded by ruins, the Kola Superdeep Borehole was a scientific project undertaken by the Soviet Union in the 1970s to better understand the Earth’s crust.

The crust is the earth’s outermost layer. It is a thin shell in comparison to the earth’s dimension—about 30 to 50 kilometers thick and is made of light rocks such as granite and basalt. The crust floats above the Earth’s mantle, the semi-solid mass of molten rocks that make up the bulk of the planet. Underneath the mantle is believed to lie a solid core of iron.