What Is Really Hiding in the Deepest Hole on Earth?

Ever heard of the Kola Superdeep Borehole? The deepest hole on earth? Imagine if science decided to speed-run digging straight into the Earth’s crust just to see what’s up (or down). It turns out that when you drill over 7.5 miles deep, things get interesting.

We’re talking about boiling rocks, microscopic fossils that have been chilling for billions of years, and heat so intense that it literally forced scientists to stop digging. So, what exactly is lurking in the deepest hole on Earth? Spoiler: It’s not aliens (sadly), but the truth is just as mind-blowing.

A Race for First Place

In 1958, the U.S. launched Project Mohole to drill into Earth’s mantle off Guadalupe Island, Mexico. Backed by the National Science Foundation, the project reached 601 feet into the seabed. This was before funding was cut in 1966 by the U.S. House of Representatives.

In 1970, the Soviets began drilling the Kola Superdeep Borehole in Murmansk, Russia, near the Norwegian border. Their attempt was more successful, reaching deeper into Earth and uncovering samples that continue to amaze scientists.

Early Drillings for the Deepest Hole on Earth

Drilling at Kola SG-3 began on May 24, 1970, using the Uralmash-4E rig. It was modified to reach a depth of 7,000 meters (23,000 feet). In 1974, the purpose-built Uralmash-15000 rig was introduced to target a depth of 15,000 meters (49,000 feet). On June 6, 1979, Kola SG-3 broke the world depth record previously held by the Bertha Rogers hole in Oklahoma. It reached 9,583 meters (31,440 feet). By October 1982, the hole had reached 11,662 meters (38,261 feet).

The drilling project continued intermittently for 24 years.

The Kola Superdeep Borehole, located on Russia’s Kola Peninsula, reached a depth of over 7.5 miles (12 kilometers). It surpassed the deepest point of the ocean, which is nearly 6.8 miles (11 kilometers) deep.

The Second Hole

Drilling of the second hole began in January 1983. It started at a depth of 9,300 meters (30,500 feet) from the first hole. In 1983, the drill surpassed 12,000 meters (39,000 feet), but drilling was paused for about a year for scientific and celebratory visits.

This break may have contributed to issues when drilling resumed. On September 27, 1984, after reaching 12,066 meters (39,587 feet), a 5-kilometer (3.1-mile) section of the drill string twisted off and was left in the hole. Drilling resumed in September 1986, 7,000 meters (23,000 feet) from the first hole.

The Other 3 Holes

The third hole reached 12,262 meters (40,230 feet) in 1989, with expectations to reach 13,500 meters (44,300 feet) by 1990 and 15,000 meters (49,000 feet) by 1993. However, a breakdown occurred in June 1990 at 12,262 meters (40,230 feet).

Drilling of the fourth hole began in January 1991 from a depth of 9,653 meters (31,670 feet). However, it was stopped in April 1992 at 11,882 meters (38,983 feet).

The fifth hole started in April 1994 from 8,278 meters (27,159 feet), but drilling was halted in August 1994 at 8,578 meters (28,143 feet) due to lack of funds, and the well was mothballed.

What Did They Discover?

During drilling, scientists discovered much higher temperatures than expected, prompting an update to the Earth’s interior temperature map. They were also surprised to find no transition from granite to basalt, challenging the existence of the “Conrad discontinuity” predicted by seismic surveys.

Another unexpected discovery was liquid water much deeper than anticipated, with saline water-filled cracks showing that the crust is not as dense as believed and allowing fluids to flow. Researchers speculated the water may have been squeezed out of rock crystals due to high pressure.

Even more exciting was the discovery of biological activity at 4.4 miles (7 kilometers), where they found fossils of single-celled marine organisms dating back 2 billion years, surprisingly intact despite extreme conditions.

Even More Discoveries

As engineers drilled past 14,800 feet (4,511 meters), they found the rock had much more porosity and permeability, making it behave more like plastic than solid rock due to the extremely high temperatures. This made further drilling nearly impossible.

The temperatures exceeded the capabilities of their equipment, and although the Soviets continued until 1992, they never surpassed the depth reached in 1989. The drilling effort was ultimately discontinued, falling short of their 9.3-mile (15-kilometer) goal, and the site was officially shut down and sealed in 2005.

Despite these findings, scientists noted that the Earth’s center is nearly 4,000 miles (6,400 kilometers) away, meaning the 7.5-mile depth barely scratched the surface.

Is it Possible to Dig Deeper?

Yes, digging deeper is possible eventually. However, it depends on two critical factors: temperature and borehole stability, which are influenced by stress, strain, and drilling fluid composition. With temperatures predicted to reach as high as 500°F (250°C), advanced technology will be required to tackle the challenge.

Reaching Earth’s mantle, located about 25 miles (40 kilometers) below the surface, would be a major scientific breakthrough. Harms explains that drilling into the mantle would provide insights into the boundary between the crust and mantle, which is still debated.

Scientists seek fresh samples to understand how the crust and mantle interact, how fluids and magma escape into the crust and hydrosphere, and how they feed the biosphere. This boundary, known as the Moho Discontinuity, remains a key objective for understanding Earth’s evolution.

Is It Still Ongoing?

Drilling ended in 1995 due to a lack of funding. The scientific team was transferred to “Kola Superdeep,” downsized and tasked with studying the exposed section. In 2007, the team was dissolved, and the equipment was transferred to a private company, with some of it being partially liquidated.

In 2008, the company was liquidated due to unprofitability, and the site was abandoned. It is still visited by sightseers, who report that the structure over the borehole has been partially destroyed or removed.

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