Beneath this rusty old metal cap lies some of our world's deepest mysteries. Though it measures just 9 inches in diameter, the hole beneath the cap extends 40,230 feet under the Earth, or 7.5 miles. That's roughly a third of the way through the Baltic continental crust. It's the deepest borehole in the world.

The Kola Superdeep Borehole was drilled between 1970 and 1994 in a Cold War-era attempt by the Soviets to beat the United States in a race to drill to the center of the Earth — or to get as close to the center as possible. Though the space race stole all the headlines, this less-publicized subterranean quest was equally as competitive. The mysteries that it unearthed are still being analyzed today.

Before the hole was drilled, geologists could only hypothesize about the composition of the Earth's crust. Needless to say, the amount of geological data produced by the project was unprecedented. Mostly, it revealed just how little we really know about our planet.

For instance, one of the most surprising findings was the absence of the transition from granite to basalt at a depth between 3 and 6 kilometers below the surface. Previously, scientists had used seismic waves to glean information about the composition of the crust. They had discovered that a discontinuity existed at this depth, which they assumed was due to a transition in rock type. But the borehole drillers found no such transition; instead they found only more granite. It turns out that the discontinuity revealed by the seismic waves was actually due to a metamorphic change in the rock, rather than a change in rock type. It was a humbling realization for theorists, to say the least.

Even more surprising, the rock had been thoroughly fractured and was saturated with water. Free water was not supposed to exist at such depths. Geologists now surmise that the water consists of hydrogen and oxygen atoms that were squeezed out of the surrounding rock by enormous pressure, and is retained there due to a layer of impermeable rock above.

Researchers also described the mud that flowed out of the hole as "boiling" with hydrogen. The discovery of such large quantities of hydrogen gas was highly unexpected.

By far the most riveting discovery from the project, however, was the detection of microscopic plankton fossils in rocks over 2 billion years old, found four miles beneath the surface. These "microfossils" represented about 24 ancient species, and were encased in organic compounds which somehow survived the extreme pressures and temperatures that exist so far beneath the Earth.

The final mystery revealed by the borehole was the reason drilling operations had to be abandoned. Once the drill reached depths in excess of about 10,000 feet, the temperature gradient suddenly began to increase unexpectedly. At the hole's maximum depth, temperatures skyrocketed to 356 degrees Fahrenheit, which was much higher than the 212 degrees Fahrenheit originally predicted. The drill was rendered useless at such temperatures.

The project was officially closed down in 2005, and the site has since fallen into disrepair. The hole itself was welded shut by the rusted metal cap that today covers it, as if to permanently hide the hole's many mysteries from the surface world. 

Though the hole's depth is impressive, it's a small fraction of the distance to the center of the Earth, which is estimated to be nearly 4,000 miles deep. By comparison, the Voyager 1 spacecraft, which has reached the outer layers of our solar system, has relayed information from over 10 billion miles away. The human race truly understands less about the ground beneath its very feet than it does about the cosmos that abound. It's humbling to realize just how much mystery still exists right here on our little blue world.

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