What Did Radar Discover Beneath the Mare Tranquillitatis Pit on the Moon?

In 2024, radar scans from a NASA lunar orbiter confirmed that the Mare Tranquillitatis pit on the Moon does not simply drop and stop — it opens into a hidden underground lava tube tens of feet wide, making it the only lunar pit ever directly verified by radar to connect to a cave system.

What Is the Mare Tranquillitatis Pit?

The Mare Tranquillitatis — Latin for “Sea of Tranquility” — is one of the Moon’s most historically significant regions. It is the same ancient lava plain where Apollo 11 made humanity’s first crewed lunar landing in 1969. Scattered across this vast basaltic landscape are a number of mysterious surface pits, depressions formed when the roofs of ancient underground lava tubes partially collapsed billions of years ago.

One of these pits became the focus of an extraordinary 2024 study. Researchers trained radar instruments aboard a NASA lunar orbiter directly at the pit, attempting to see through the surface and into whatever lay beneath.

What the Radar Revealed

The results were unprecedented. Rather than a simple vertical shaft that terminates in rubble, the radar data revealed a horizontal conduit branching off from the base of the pit — a lava tube, intact and tens of feet wide, running beneath the Moon’s surface. This subterranean passage had survived billions of years without collapsing.

Lava tubes form when the outer shell of a flowing lava stream cools and hardens while molten rock continues moving through the interior. Once the eruption ends and the lava drains, a hollow tunnel remains. On Earth, we see these structures in places like Hawaii and the Canary Islands. On the Moon, with its lower gravity and ancient volcanic history, such tubes are theorized to potentially reach enormous dimensions — and this radar confirmation is the first direct observational proof that at least one survives as an accessible underground space.

The Temperature Discovery That Changes Everything

Perhaps the most staggering detail involves temperature. The Moon’s surface is one of the most thermally extreme environments in the solar system. During the lunar day, surface temperatures can climb above 250°F (121°C). At night, they plunge to nearly -300°F (-184°C). That is a swing of roughly 500°F across every single lunar day — a cycle that repeats approximately every 29.5 Earth days.

Inside the lava tube connected to the Mare Tranquillitatis pit, however, scientists measured something remarkable: a stable temperature of approximately 63°F (17°C). That figure holds with less than two degrees of variation across an entire lunar day. Scientists have described the environment, only half-jokingly, as “sweater weather on the Moon.”

The tube’s thick rock ceiling acts as natural insulation, blocking solar radiation, micrometeorite impacts, and the wild thermal swings that make the lunar surface so hostile to life and equipment alike.

Why This Matters for Human Exploration

The implications for future lunar exploration are profound. One of the greatest engineering challenges of building a permanent lunar base is shielding astronauts and equipment from radiation and temperature extremes. A naturally stable, pre-existing underground structure addresses both problems simultaneously — without requiring the construction of artificial shelters.

NASA’s Artemis program aims to return humans to the Moon and eventually establish a sustained presence there. Lava tubes like the one confirmed beneath Mare Tranquillitatis could serve as ready-made habitats, requiring far less infrastructure than surface-based alternatives. They may also trap water ice and other volatiles that would be essential for long-duration human missions.

A Window Into Lunar Geology

Beyond its practical significance, this discovery is a window into the Moon’s volcanic past. The existence of an intact lava tube billions of years after the Moon’s volcanic activity ceased tells scientists that the lunar subsurface is far better preserved than previously understood. Studying these structures could help reconstruct the timeline and scale of ancient lunar volcanism — and refine our models of how rocky worlds like Earth and Mars evolved.

The Mare Tranquillitatis pit has gone from a geological curiosity to one of the most scientifically and practically important locations on the Moon. And radar — a technology more often associated with weather forecasting and air traffic control — is what finally let us see inside it.