The Moon — Lunar Lava Tubes: Underground Cities?
The Moon — Lunar Lava Tubes: Underground Cities?
What Are Lunar Lava Tubes?
Lava tubes are natural tunnels formed when the outer surface of a lava flow solidifies while molten lava continues to flow through the interior. When the eruption ends and the molten interior drains out, it leaves behind an empty tube — sometimes hundreds of kilometres long and tens of metres to hundreds of metres across.
Lava tubes are found on Earth, Mars, Venus, and the Moon. On Earth, some lava tubes are used for caves, tourism, and scientific study. On the Moon, the extremely low gravity (one-sixth of Earth's) and the absence of erosion by water or wind have allowed lava tubes formed billions of years ago to survive intact to the present day.
The Scale of Lunar Lava Tubes
This is where lunar lava tubes become extraordinary: because the Moon's gravity is so much weaker than Earth's, lunar lava tubes can be vastly larger than their terrestrial equivalents while remaining structurally stable.
Models and observations suggest that lunar lava tubes may be:
- Several kilometres wide (compared to typical terrestrial lava tubes of a few metres to a few tens of metres)
- Hundreds of kilometres long
- Tall enough that the roofs could support the weight of a few hundred metres of rock without collapsing
A 2017 study by researchers at Purdue University calculated that lunar lava tubes could potentially be up to 5 km wide and remain stable under the Moon's gravity. To put this in context: a tube 5 km wide and of any significant length could enclose a volume larger than many of Earth's largest cities.
The Evidence for Their Existence
Several lines of evidence confirm that large lunar lava tubes exist:
Sinuous rilles: Long, winding channels visible on the lunar surface are interpreted as the surface traces of ancient lava flows and collapsed lava tubes. Hadley Rille, visited by Apollo 15, is a prominent example.
Pit craters (skylights): Since 2009, the Lunar Reconnaissance Orbiter has identified hundreds of circular pits on the lunar surface — vertical shafts that appear to open into larger subsurface voids. These "skylights" are the lunar equivalent of where a lava tube roof has collapsed, and they provide direct evidence that large subsurface voids exist.
Gravity anomalies: GRAIL mission gravity data revealed subsurface mass deficits consistent with large empty tubes beneath some regions of the lunar surface.
Scientific Significance
The scientific interest in lunar lava tubes is genuine and substantial:
- They represent potentially habitable spaces for future human lunar settlements — naturally shielded from radiation, micrometeorites, and extreme temperature variations by the overlying rock
- The interiors would maintain stable temperatures (approximately -20°C) compared to the extreme surface temperature swings
- Their scale could support large human installations without artificial pressurisation structures
- China, NASA, and ESA have all included lava tube exploration in long-term lunar settlement planning
The Conspiracy Dimension
The discovery of kilometre-scale empty voids beneath the lunar surface — large enough to contain cities, shielded from Earth-based observation, structurally stable and temperature-regulated — has inevitably become part of the artificial moon and hidden base literature.
The argument: if advanced extraterrestrial beings constructed a hollow Moon or established bases on the Moon, the lava tubes — large, concealed, protected — would be the natural location for such facilities. The fact that these enormous subsurface voids exist and are effectively invisible from Earth has been incorporated into multiple frameworks that propose current or historical non-human habitation of the lunar interior.
The scientific position is clear: the lava tubes are natural formations produced by ancient volcanism, consistent with the Moon's geological history. Their extraordinary scale is a consequence of lunar gravity, not design. Their existence is fascinating for space exploration planning, not evidence of artificial construction.
