The story of how humanity ended up underground begins not with digging, but with something far more surprising: walking into openings that already existed. Long before the first tunnel was carved, natural caverns formed by volcanic flows, dissolving limestone, and geological forces were already present across the planet, waiting. About one million years ago, early hominins made a decisive breakthrough. At Wonderwerk Cave in South Africa’s Northern Cape Province, researchers led by Michael Chazan of the University of Toronto and Leora Kolska Horwitz of the Hebrew University found sediment evidence of burned bone and ash roughly 30 meters inside the cave.
Published in 2012, this represented the earliest secure evidence of controlled fire use deep inside a subterranean space, a sign that Homo erectus had learned to make the dark habitable. By roughly 176,500 years ago, Neanderthals had gone further. In Bruniquel Cave in southwestern France, a team led by Jacques Jaubert of the University of Bordeaux published findings in Nature showing that Neanderthals had transported and arranged about 400 broken stalagmites weighing over two metric tons into large circular structures 336 meters inside the mountain, with evidence of internal hearths. No practical explanation has ever fully accounted for the structures, suggesting something about the underground itself held deep meaning.
The leap from using caves to deliberately excavating them occurred at least 43,000 years ago in Eswatini at a site called Lion Cavern on Bomvu Ridge. Excavated in the late 1960s by archaeologist Peter Beaumont and Professor Raymond Dart, the site revealed that Middle Stone Age miners had driven tunnels into an iron-bearing hillside to extract red ochre. Over 300,000 specialized mining tools, including dolerite hammerstones and stone picks, remained behind. The oldest known deliberately dug tunnels on Earth were not for shelter or water but for red pigment used on bodies, tools, and the dead.
During the Neolithic period, Europeans turned the underground into an industrial workplace. At Grimes Graves in Norfolk, England, first excavated by Canon William Greenwell between 1868 and 1870, over 430 shafts plunge 12 to 14 meters through chalk. At the bottom, miners crawled into horizontal galleries barely a meter high to chip out high-quality floor stone using picks made from red deer antlers and shovels from cattle shoulder blades. They left pillars of uncut chalk to support the roof, a technique still used in modern mines.
The same pattern appeared at Spiennes in Belgium, discovered in 1867 by Alphonse Briart and François Léopold Cornet. This UNESCO World Heritage site covers over 100 hectares, with shafts up to 16 meters deep and complex subterranean networks, worked continuously for over 2,000 years across multiple generations of excavation. Around 4,000 BC in Malta, builders carved a different kind of underground space. The Hypogeum of Ħal Saflieni, discovered accidentally during construction work in 1902, is a three-level subterranean complex of about 500 square meters, hand-carved with flint, obsidian, and red deer antler picks.
It contains interconnected chambers and the Oracle Chamber, which produces a deep acoustic resonance at roughly 110 hertz, a frequency shown in neuro-acoustic studies to affect emotional and language centers of the brain. This may have been deliberate architecture designed to alter human experience inside the earth. The introduction of qanats transformed civil engineering in arid landscapes. In Persia, modern-day Iran, these subterranean water channels involved sinking a deep vertical mother well into an aquifer and digging a gently sloping horizontal tunnel to a settlement on the plains.
The water flows by gravity alone, avoiding the extreme evaporation that destroys surface canals. The Kasabe Qanat in Gonabad has a mother well over 300 meters deep and gallery systems stretching over 33 kilometers, built about 2,700 years ago and still functioning. The Zarch Qanat near Yazd runs over 71 kilometers with 2,115 vertical shafts. In 2016, UNESCO inscribed 11 qanat systems as World Heritage sites, including a rare double-decked qanat with two independent channels at different depths.
Building these systems required precise surveying over long distances, representing an early application of mathematics to the underground. King Hezekiah faced a mortal threat around 701 BC when the Assyrian king Sennacherib marched toward Jerusalem. With the city’s only water source outside its walls, Hezekiah ordered his workers to dig through 533 meters of solid limestone to connect the Gihon Spring to the Pool of Siloam inside the city. The two teams dug from opposite ends, navigating toward each other by listening for picks through the rock.
The Siloam inscription, discovered in 1880 by a student named Jacob Eliahu, records the breakthrough in Paleo-Hebrew script. A more ambitious engineering feat followed about 170 years later on the Greek island of Samos. Around 530 BC, the engineer Eupalinos of Megara was commissioned by the tyrant Polycrates to build a water tunnel through Mount Kastro. At 1,036 meters long through solid limestone, the tunnel was dug from both ends, but Eupalinos designed a deliberate angular turn near the center, widening the target area.
When the two teams met, the horizontal error was less than 60 centimeters, and the vertical error was nearly zero. The historian Herodotus later called it one of the three greatest engineering works of the Greeks. Military tunneling changed the nature of warfare. During the siege of Veii in 396 BC, the Roman dictator Marcus Furius Camillus ended a standoff by ordering soldiers to dig a tunnel directly under the city’s walls, emerging through the floor of the temple of Juno inside the citadel.
Siege tunneling became standard practice, leading to underground battles. At Dura-Europos in eastern Syria, around 256 AD, Persian sappers dug beneath the city’s Tower 19, but Roman defenders dug a countermine. The Persians ignited a mixture of bitumen crystals and sulfur, pumping sulfur dioxide gas through the passage. In 2009, archaeologist Simon James of the University of Leicester reanalyzed the bodies found there and confirmed the deaths of 19 Roman soldiers and one Persian sapper, the earliest verified chemical weapon attack in recorded history.
The Romans industrialized tunneling. Between 41 and 52 AD, Emperor Claudius ordered the drainage of Lake Fucino in central Italy through a 5. 6-kilometer tunnel driven beneath Mount Salviano with over 32 vertical shafts, some exceeding 120 meters deep, allowing more than 80 teams to work simultaneously. Around 30,000 laborers contributed over 11 years.
The tunnel held the record as the longest on Earth for more than 1,800 years. At Naples, the architect Lucius Cocceius Auctus carved the Crypta Neapolitana through volcanic tuff in 37 BC, a 700-meter road tunnel. The philosopher Seneca traveled through it and complained of dust and darkness, leaving the first recorded unfavorable tunnel review. Even the Romans made errors.
Around 152 AD, the hydraulic engineer Nonius Datus was summoned after two teams digging an aqueduct tunnel near Saldae, in what is now Béjaïa, Algeria, missed each other inside the mountain. He corrected the alignment with a linking gallery, and the episode was inscribed on a stone cippus discovered in 1866. For centuries, fire-setting remained the primary excavation method. Then on February 8, 1627, at the Oberlager mine in Štiavnica, now Banská Štiavnica in Slovakia, mining engineer Caspar Weindl packed black powder into hand-drilled bore holes and detonated them before imperial mining officials.
It was the first documented use of explosives underground. In 1679, engineer Pierre-Paul Riquet used the technique to blast the 165-meter Malpas tunnel on the Canal du Midi, the first civil transport tunnel built with explosives. Soft ground remained unconquered until the engineers of the Thames and London learned from a worm. In 1818, French-born engineer Marc Isambard Brunel patented the tunneling shield after studying Teredo navalis, the naval shipworm, which bores through timber while secreting a hard calcified lining behind itself.
Brunel built a 90-ton cast-iron frame with 36 modular cells. The Thames Tunnel took 18 years to build, with the river breaking through five times. On January 12, 1828, a flood killed six workers and nearly killed Brunel’s son, Isambard Kingdom Brunel, who was swept up a shaft unconscious. At least 10 men died overall.
The tunnel opened on March 25, 1843, and remains in use today as part of the London Overground. Mountain tunneling accelerated after 1857 with the Mont Cenis Tunnel, later called the Fréjus Tunnel, connecting France and Italy. Chief engineer Germain Sommeiller introduced a pneumatic compressed-air rock drill in 1861, and Alfred Nobel’s dynamite arrived mid-construction. On Christmas Day 1870, the two teams met beneath the mountain, with a horizontal error under 30 centimeters across a 12.
2-kilometer drive. Sommeiller died of exhaustion on July 11, 1871, two months before the tunnel opened on September 17. The project broke the 1,800-year record held by Claudius’s tunnel and proved that compressed-air drills could conquer entire mountain ranges. In 1953, engineer James Robbins built the first modern tunnel-boring machine for the Oahe Dam diversion tunnels in South Dakota.
His 1956 rolling disc cutter crushed rock through tensile fracturing, using a fraction of the energy of previous methods. The Channel Tunnel between England and France, built from 1988 to 1994, used 11 specialized machines to bore 50. 45 kilometers of tunnel, with 37. 9 kilometers beneath the sea.
On December 1, 1990, workers Graham Fagg and Philippe Cozette shook hands through the pilot bore. The idea had first been proposed in 1802, but it took 188 years to complete. The Gotthard Base Tunnel in Switzerland opened in 2016, stretching 57. 09 kilometers and reaching 2,450 meters beneath the Alps.
The longest and deepest traffic tunnel on Earth, it took 17 years to build, with rock temperatures reaching 46°C at the deepest points and nine workers dying during construction. The longest tunnel record kept falling and the engineering kept advancing, but the core drive never changed. Tunneling was independently developed on every inhabited continent, for ochre, flint, qanats, copper, defense, and transportation. The probability that humans would develop some form of deliberate tunneling was effectively inevitable, driven by universal needs that the surface alone could not satisfy.
In Cappadocia, modern-day Turkey, the underground city of Derinkuyu descends 85 meters into volcanic rock across 18 levels, with room for 20,000 people, stables, churches, ventilation ducts, and 500-kilogram stone doors that could seal the entrance from within. It was rediscovered accidentally in 1963, when a local resident knocked down a basement wall during renovations and found one room behind it, then another, and another. The earth had already carved its own hollows, and humans kept finding reasons to make more of them.
Every tunnel under a city, river, or mountain today exists because some 43,000 years ago, someone looked at a hillside, picked up a stone, and decided that whatever was inside was worth digging for.