In the Arctic wilderness near Russia's border with Norway, sealed beneath a welded steel cover bolted into the ground, sits the deepest hole human beings have ever drilled into the Earth — a 9-inch-wide shaft called the Kola Superdeep Borehole, plunging 7.6 miles down through 2.7 billion years of rock, abandoned in 1992 when the temperature at the bottom reached 356°F and the rocks themselves began to flow like plastic

Across the same two decades in which the United States and the Soviet Union were competing to send rockets and astronauts further and further out from Earth, a much quieter and less photogenic version of the same competition was being conducted in the opposite direction. The two superpowers had each launched, in parallel, ambitious scientific programmes aimed at drilling as deep into the planet as their respective engineering bases could manage. The American programme — Project Mohole, conceived in 1958 — aimed to drill through the relatively thin oceanic crust off the coast of Mexico and reach the upper mantle. It was cancelled in 1966 due to funding and political difficulties, having drilled approximately 183 metres beneath the seafloor. The Soviet programme, which began formal planning in the early 1960s and started drilling on 24 May 1970, took a different approach: instead of attempting to drill through the thin oceanic crust, the Soviet team would drill straight down through the much thicker continental crust on the Kola Peninsula in the Russian Arctic — a region selected because it offered exposed Precambrian shield rock approximately 2.7 billion years old, providing geophysicists with a chance to sample some of the oldest accessible material on the planet’s surface.

According to a comprehensive reference summary of the Kola Superdeep Borehole’s full operational history and the engineering challenges it encountered, the project employed a sequence of progressively more specialised drilling rigs across its 24-year operational life. The initial rig, the Uralmash-4E, was a slightly modified oil-drilling unit and was used until 1974, when it was replaced by the purpose-built Uralmash-15000, named after the new target depth of 15,000 metres. The hole itself — designated SG-3, the third in a series of Soviet superdeep boreholes — was nine inches in diameter at the bottom, narrowing slightly with depth, and was drilled using a turbine drill bit driven by pressurised mud flowing down the drill pipe. On 6 June 1979, SG-3 surpassed the previous world depth record held by the Bertha Rogers hole in Oklahoma. By June 1989, it had reached 12,262 metres — a depth no human-drilled hole had ever reached before, or has reached since.

Why the drilling stopped

The fundamental problem that ultimately ended the project was not engineering ambition but the behaviour of the rocks themselves at the relevant depths. As reported by Scientific American’s analysis of why the Kola hole was eventually abandoned, the temperature at the bottom of the borehole had been predicted by Soviet geophysicists to reach approximately 100 degrees Celsius — uncomfortable but manageable with existing drilling technology. The actual temperature at 12,262 metres turned out to be approximately 180 degrees Celsius, or 356 degrees Fahrenheit, almost twice the predicted value. At those temperatures, the rocks at the bottom of the hole no longer behaved like the solid granite the drill had been chewing through at shallower depths. The rocks became plastic — deforming under the pressure of the drilling equipment, flowing slowly back into the hole as soon as the drill bit was withdrawn, and exerting enough pressure on the drill bits themselves to deform and eventually break them.

The combination of plastic rock behaviour and unanticipated thermal gradient made further progress essentially impossible with the available technology. The Soviet team continued drilling for several more years after the 12,262-metre depth was reached, but never managed to extend the hole significantly further. The final abandonment of active drilling came in 1992, shortly after the dissolution of the Soviet Union, when the political and economic chaos that followed the collapse made continued funding for what was, by any reasonable measure, a frontier-science project impossible to maintain. The facility surrounding the borehole continued to operate at reduced capacity for several more years, conducting analyses of the cores and data already obtained, before being formally closed in stages through the late 1990s and 2000s. By 2008 the site was completely abandoned. The 27-storey tower that had originally housed the drilling operation was eventually dismantled by local scrap-metal collectors.

What the project actually found

The scientific yield of the Kola Superdeep Borehole was, in proportion to the cost and effort involved, substantial. As detailed in a comprehensive account of the borehole’s history and the unexpected discoveries it produced, the drilling team encountered several findings that overturned previously held assumptions about the structure and composition of the continental crust. The Conrad discontinuity — a long-hypothesised transition zone between granite and basalt that geophysicists had expected to encounter around six kilometres depth — turned out not to exist; the granite simply continued much deeper than anyone had predicted, with the seismic discontinuity that had been interpreted as a compositional boundary turning out to be a structural feature caused by increased rock fracturing rather than a change in rock type. The drilling mud flowing out of the borehole was found to be saturated with free hydrogen gas — bubbling continuously from the hole, in concentrations that surprised the geochemists analysing it — along with helium, nitrogen, and carbon dioxide. Liquid water was found at depths where it should not have existed under the prevailing temperature and pressure conditions. And at approximately 6.7 kilometres depth, the team recovered fossilised remains of 24 distinct species of microscopic single-celled marine organisms, encased in carbon and nitrogen compounds that had preserved them through approximately 2 billion years of geological history.

The fossils, in particular, transformed scientific understanding of subsurface biology. The organisms had lived on Earth approximately 2 billion years ago, when the rocks containing them were near the surface; the subsequent burial of those rocks to their current depth had been gradual enough, and the carbon-nitrogen preservation matrix protective enough, that the microfossils survived essentially intact across the intervening geological epochs. Per a HowStuffWorks analysis of the borehole’s discoveries and the bizarre internet mythology that subsequently developed around it, the practical reality of the hole — a quiet, sealed, 23-centimetre-wide industrial-scientific artefact in a remote corner of the Russian Arctic — became, in the years after its abandonment, the subject of a persistent internet hoax claiming that microphones lowered into the borehole had recorded the screams of the damned. The story, which appears to have originated as a Finnish newspaper joke in the 1990s, was repeated widely enough in the early days of the internet that the Kola Superdeep Borehole acquired the popular nickname “the well to hell.” No microphones were ever lowered into the hole. No screams were ever recorded. The rocks at the bottom are 180 degrees Celsius and behaving plastically. The hole itself ends at 12,262 metres in a sealed cap of rusting metal. The drilling reached approximately 0.2 percent of the distance from Earth’s surface to its centre, and then stopped. Voyager 1, launched seven years after the Kola drilling began, has by now reached interstellar space. The species, by every available measure, has found it dramatically easier to leave the planet than to drill into it.