We have flown to the Moon, but we have barely scratched our own planet. This is how humanity drills, bores and digs into the crust — for oil and gas, for heat, for water, and for the tunnels beneath our cities — and just how little of the Earth we've actually reached.
Between 1970 and 1989, Soviet scientists on the Kola Peninsula drilled the Kola Superdeep Borehole to 12,262 metres — still the deepest hole humans have ever made, and a point only about 0.2% of the way to the Earth's centre. The bit was barely into the continental crust.
They stopped not because they ran out of ambition but because the rock fought back: at that depth temperatures hit 180°C, far hotter than predicted, and the rock behaved less like solid stone and more like plastic, closing the hole as fast as they drilled. The deepest reach today belongs to oil wells like Sakhalin-1's Z-44, which extends over 12 km — but mostly sideways.
The lesson of Kola is humbling: we have sent probes to the edge of the solar system, yet the deepest we have ever physically reached into our own planet would not even get you through the crust to the mantle below.
At the tip of a kilometres-long "drill string" spins the bit — studded with polycrystalline diamond or tungsten-carbide teeth that grind through rock. As the hole deepens, crews add 9-metre pipe sections and periodically pull the whole string out to swap a worn bit, a slow ritual called "tripping."
Drilling "mud" — a engineered fluid — is pumped down the pipe and back up the hole. It cools and lubricates the bit, carries crushed rock ("cuttings") to the surface, and its weight balances the pressure of oil, gas and water trying to force their way in. Get the mud weight wrong and the well can blow out.
Above the wellhead sits the blowout preventer (BOP) — massive hydraulic rams that can seal the well in seconds if pressure surges. The failure of a BOP was central to the 2010 Deepwater Horizon disaster, and they are now the most scrutinised piece of hardware on any rig.
Modern wells no longer just go straight down. Steerable bits turn the hole horizontal, running for kilometres through a thin oil- or gas-bearing layer. Combined with hydraulic fracturing — pumping fluid at high pressure to crack the rock — this unlocked the shale boom that reshaped global energy.
Go deep enough anywhere and the rock is hot. Enhanced geothermal borrows shale's horizontal-drilling and fracturing tricks to make steam far from volcanoes, and "superhot" projects target 400°C+ rock — potentially a limitless, carbon-free heat source, covered on the Energy page.
Billions rely on drilled water wells, while scientific programs like ocean drilling pull up rock cores that record millions of years of climate and tectonics — the deepest direct samples we have of how the planet works.
Not all drilling seeks resources. Giant tunnel-boring machines — some over 17 m across, as wide as a building is tall — chew horizontally through rock and soil to carve subways, road tunnels and rail links, lining the walls with concrete segments as they crawl forward.
Millimetre-wave drilling. Startups are testing ways to vaporise rock with high-power millimetre-wave energy instead of grinding it, aiming to reach the ultra-deep, ultra-hot rock (10–20 km) where geothermal energy is effectively unlimited — depths no mechanical bit can survive.
Automation. Rigs are increasingly robotic, with automated pipe handling and AI steering the bit in real time from downhole sensor data, cutting both cost and the danger of the rig floor.
The mantle, one day. Scientists still dream of drilling clean through the crust to the mantle — the goal of the decades-long "Mohole" ambition. We can reach orbit routinely; reaching the rock 30 km beneath our feet remains one of engineering's great unfinished challenges.
> Sources: public records on the Kola Superdeep Borehole, oil & gas engineering, and geothermal drilling programs.