The Deepest Hole Ever Drilled Was Abandoned When the Earth Became Too Hot to Continue

From 1970 to 1989, Soviet scientists in the Kola Peninsula of Russia drilled the deepest hole ever dug into the Earth. The Kola Superdeep Borehole reached 12,262 meters (40,230 feet, or 7.6 miles) — deeper than the deepest part of the ocean (the Mariana Trench, at 10,994 meters).

It was meant to reach much deeper. The original goal was 15,000 meters. But at 12,262 meters, the project hit conditions that no equipment could handle.

Why They Stopped

• Temperature: 180°C (356°F) — far hotter than expected at that depth. Drill bits failed rapidly
• Plasticity of rock: At those temperatures and pressures, the rock behaved more like plastic than solid stone, deforming around drill equipment
• Pressure: Each meter of drill string was being crushed by the weight of all the equipment above it
• Cost: The project was bankrupting itself trying to make any progress at all

Surprising Discoveries

Despite the challenges, the borehole produced major scientific insights:

• Microorganisms were found at depths of 6,700+ meters — proving life exists deep within the Earth's crust
• Researchers found liquid water at depths far below where it was thought to exist, in environments with extreme heat and pressure
• The transition zone where granite is thought to give way to basalt did not exist as expected — disproving a long-held geological assumption
• Hydrogen gas was unexpectedly abundant, leaking out continuously

How Deep Is Deep?

For perspective, the Kola Superdeep Borehole reached only 0.2% of the way to Earth's center. The deepest natural caves go to about 2,200 meters; the deepest mines about 4,000 meters. Reaching the mantle, much less the core, is currently considered technologically impossible.

Today

The borehole was abandoned in 1995 due to cost. The site is now derelict, the entrance welded shut. The Russian government considered reopening it as a tourist attraction but never did.