The Marius Hills pit

The Marius Hills are a cluster of low volcanic domes in the Ocean of Storms, a huge dark plain on the Moon’s near side. They are named after Marius, a nearby crater about 41 km wide. In 2009, Japan’s Kaguya (SELENE) orbiter spotted something unusual among them: a deep, nearly round hole with steep walls, sitting inside a winding lava channel.

Scientists think the hole is a ‘skylight’, a spot where the roof of an underground lava tube caved in. A lava tube is a tunnel left behind when the outside of a lava river cools into a crust while hot lava keeps flowing inside and later drains away. That makes this little pit a possible doorway into a hidden cave, and caves like this could one day shelter astronauts.

How big is the pit?

The first report in 2009 described the hole as about 65 m across. Later, NASA’s Lunar Reconnaissance Orbiter (LRO) photographed it at a resolution of half a metre per pixel. Using pictures taken with the Sun higher in the sky, the LRO camera team measured it at about 58 by 49 m and about 40 m deep. Earlier estimates of its depth were deeper, up to around 88 m, so the exact numbers depend on which study you read.

How lava tubes form

Billions of years ago, rivers of runny lava flowed across this region. As the top of a flow cooled and hardened, it formed a roof, while molten rock kept moving underneath like water in a pipe. When the eruption ended, the lava drained out and left an empty tunnel. Kaguya’s radar hinted at empty spaces under nearby channels too, and gravity maps from NASA’s GRAIL mission show places with ‘missing mass’ that could be more tubes.

A future Moon base?

The Moon’s surface is a harsh place. There is no air to block radiation from the Sun and space, tiny meteorites rain down, and temperatures swing wildly between day and night. Deep inside a cave, the rock overhead would block radiation and meteorites, and the temperature would stay much steadier, as it does in caves on Earth. Nobody yet knows whether this pit leads into an open tunnel that people could enter, but it is one of the top places scientists would like to explore.

On the 3D Moon

On the 3D Moon you can find this deep pit in the Marius Hills, in the Ocean of Storms on the Moon’s western near side. Open its card to learn why a hole in a lava cave could become a future Moon base.