Could Humans Build a City Inside a Mars Lava Tube?

Short answer: possibly one day, but nobody has been inside a Martian lava tube yet. Orbiters have photographed pits and possible cave skylights on the volcanoes of Mars, and a 2020 review suggests Martian lava tubes could be far wider than any on Earth. Underground, a crew would get natural protection from radiation, temperature swings and dust. What no one knows is whether a given tube is intact, reachable, and stable enough to build in.

That open question is the ground The Red Dominion builds on. In Book 3 of SOLEN: The Eden Archive, Carter is “walking twenty-two kilometres of lava tube corridors every morning, pressing his hand against basalt,” building “a city inside a planet that accepts him because he builds with it and not against it.” This post separates the real science of living in lava tubes from the story.

What is a lava tube?

A lava tube is a natural tunnel left behind by flowing lava. NASA’s Jet Propulsion Laboratory describes how some lava “can squeeze underneath older rock and lift it up so it can continue to flow underground.” When the eruption stops, the lava can drain out, leaving an empty tube. These tubes are common on Earth, and JPL says they “may criss-cross the giant volcanoes of Mars.”

On Earth, cavers have mapped tubes such as Kazumura in Hawaiʻi, Corona in the Canary Islands and Undara in Australia. Where part of a tube’s roof falls in, it leaves a pit or a “skylight” that opens into the tunnel below. From orbit, those holes are the clue planetary scientists look for.

Have lava tubes been found on Mars?

Strong candidates have, though none has been entered. In 2007, NASA’s Mars Odyssey orbiter found seven possible cave entrances on Arsia Mons, nicknamed the “Seven Sisters.” They are very dark, nearly circular holes about 100 to 250 metres across. The team’s paper, first presented at the Lunar and Planetary Science Conference and then in Geophysical Research Letters, estimated minimum depths of 68 to 130 metres.

The best evidence was their temperature. From day to night, the holes changed temperature only about one-third as much as the ground around them: cooler in the day, warmer at night, as you would expect from an opening into the subsurface. The researchers were careful, though. They could not tell whether the holes are deep vertical shafts or entrances to wide caverns, and they called the sites, at more than 5.7 km elevation, poor candidates for human habitation.

Higher-resolution images have since added more. In an image JPL released in 2020 from the HiRISE camera on NASA’s Mars Reconnaissance Orbiter, a collapsed lava-tube ceiling on Arsia Mons left a pit about 50 metres across. JPL notes that the tube beneath is likely at least that big, “much bigger than similar caves on the Earth,” and that HiRISE cannot see inside because the pits are in shadow whenever the orbiter passes over.

How big could a Martian lava tube be?

Big enough to make the idea of a city less far-fetched. In a 2020 review in Earth-Science Reviews, Sauro and colleagues measured collapse chains on Earth, Mars and the Moon. The collapses on Earth reach a maximum width of about 20 to 30 metres. On Mars they range from 40 to 400 metres, and on the Moon from 500 to 900 metres. The average volume of the Martian collapses, per unit length, is almost 80 times larger than on Earth.

Gravity is a big part of the reason. Lower gravity favours higher eruption rates and longer, thicker lava flows, and it lets wider conduits form while staying within the limit at which a roof holds. The same review adds a caution: intact sections of tube “surely exist” on Mars, but the biggest tubes show more frequent collapses, and wind-blown dust may have partly filled them. The authors rank the Moon, not Mars, as the most promising body for intact tubes.

Why would people live underground on Mars?

Mostly for shelter. Mars has no global magnetic field and only a thin atmosphere. Over about 300 days in 2012 and 2013, the radiation detector on NASA’s Curiosity rover measured an average dose equivalent of 0.64 ± 0.12 millisieverts per day from galactic cosmic rays at Gale Crater (Hassler et al., 2014, record on NASA’s Technical Reports Server). Combined with cruise data, the team estimated about 1.01 sieverts for one possible round trip: 180 days each way and 500 days on the surface.

Rock and soil overhead cut that dose, but not in a simple way. A 2022 modelling study (Zhang et al., Journal of Geophysical Research: Planets) found that neutron flux and effective dose actually peak around 30 cm below the surface, so a thin cover can make things worse. To keep the annual effective dose to 100 mSv, the required regolith depth ranged from about 1 to 1.6 metres. A lava tube could, in principle, supply far more cover than that without anyone digging, if its roof is thick and sound.

There is a little field data, from Earth. A study accepted by the Journal of the Washington Academy of Sciences (Paris et al., 2019) ran 30 radiation measurements at analogue lava tubes in California, New Mexico and Arizona. Readings inside averaged about 82 percent lower than outside. The same team identified three candidate lava tubes near Hadriacus Mons, a low volcano on the northeastern edge of the Hellas basin, the region where the SOLEN series is set. These are Earth measurements and orbital candidates, not a test on Mars.

What would it take to build a city in one?

Far more than finding a hole. Based on the sources above, the open problems include:

• Access. The best-studied skylights are high on the Tharsis volcanoes. The Seven Sisters team called them too small and too high in elevation to reach with landing technology at the time.
• Seeing inside. Orbiters cannot image the shadowed interiors, so the size, floor and roof of any Martian tube are inferred from collapses on the surface.
• Roof stability. The 2020 review links the widest Martian tubes to more frequent collapses. Every section would need surveying before anyone moved in.
• Air. A tube is shelter, not a sealed building. Settlers would still need pressurised habitats or sealed sections inside it.

Where does the science end and the story begin?

The novel takes the step the evidence cannot yet take: an intact, accessible tube, mapped and lived in. The rest of the premise, that the safest Martian home may be underground in volcanic rock, is a serious idea in planetary science. The book is also about who governs that city. When Earth proposes making the colony a legal subsidiary of the NovaSeed Global Initiative, the book page puts it plainly: “The colony votes thirty-nine to two to refuse it.”

Carter’s path to the tunnels starts long before Mars. In Carter: The First Record, “He spent fifteen years learning to listen to broken things.” For SOLEN’s in-world record of how Mars corrected him within six hours of landing, read Why Everything Carter Learned Was Wrong. More explainers are collected on the Mars Science hub, and if you came here from Kim Stanley Robinson, see our guide to books like Red Mars. New readers can read chapter one free or follow the reading order from Book 1.

The Red Dominion and the SOLEN: The Eden Archive series are fiction. Scientific statements in this post are linked to their sources below; story details and quoted lines come from the books’ own descriptions and are labelled as the novel’s.