Although we aren’t treated to such exciting sights when we look up at the Moon today, we do see the aftermath of its volcanic era. The familiar face of the Moon has dark areas all over it, which scientists call plains, seas, or maria. These are made of basaltic rock, which erupted from volcanoes and settled into low-lying areas like impact craters.
In 2009, the Japanese lunar orbiter Kaguya observed another remnant of the Moon’s past volcanism. The spacecraft imaged a large pit that is too deep for its width to be an impact crater. Instead, it appears to have been formed by lava.
Back in its volcanic days, the Moon’s low gravity allowed much larger volumes of lava, especially low-viscosity ballistic lava, to flow from volcanic eruptions across much greater distances than we see with lava flows on Earth. Sometimes, a flow would carve out a channel in the rock below, and the top layer of lava would cool down. The remaining magma would continue to flow beneath in a tube-shaped passage until all the magma eventually drained out, leaving the tube behind.
These subsurface tubes reveal themselves to orbiting spacecraft when a portion of the tube collapses due to a nearby impact or some other disturbance, leaving a hole called a skylight that can be seen from above by spacecraft like Kaguya.
Since this first skylight discovery, various lunar orbiters have found more than 200 others on the maria (the flat, dark plains) of the lunar nearside. They range in diameter from about 5 meters (16 feet) to more than 900 meters (2,950 feet) and are believed to lead to particularly enormous lava tubes. The biggest lava tubes we’ve found on Earth, in Kazumura Cave on the Big Island of Hawai’i, are about the size of a typical tunnel through which you’d drive a car. On the Moon, some are 300 to 700 times that size. There are lava tubes tall enough to fit the world’s tallest building inside them, and others are large enough to contain a whole city.