One of the most prominent bits of recent space news was the discovery of a “city-sized” underground tunnel on the moon. Scientists working for JAXA, Japan’s space agency, made this landmark discovery using data from Kaguya (also known as SELENE), a lunar orbiter.

Much of the excitement surrounding the announcement has, of course, been focused on its potential for human settlement. An underground space would have the enormous advantage of shielding its occupants from the dramatic temperature swings of a world without an atmosphere, as well as from the space radiation that constantly assaults the surface of a world without a magnetic field.

There is plenty of discussion about that, but what hasn’t been discussed is this: what exactly is this tube?

If you look up at a full Moon, you will see a largely light-colored face partly covered by darker patches. The lighter-colored areas are the lunar highlands, which are rugged, hilly and far more cratered. The dark-colored areas, on the other hand, are flat plains. They are called “maria,” derived from a Latin word for “sea,” a label that harkens back to a time when observers still thought there was liquid water on the lunar surface. Indeed, many of the maria’s names – Sea of Clouds, Sea of Serenity, Sea of Tranquility, to name a few – strongly evoke the aquatic and the nautical.

Indeed, the maria formed from a flood, of sorts. They are actually the remnants of ancient volcanic eruptions that flooded low-lying areas. The resulting pools of lava eventually cooled and formed the dark terrain that we see today. The maria have relatively few craters because they are newer than the highlands, and have had much less time to be exposed to bombardment from the sky.

Now, how is any of this relevant to the recent discovery? The tunnel is located an area known as the Marius Hills, which are located within what is known as the Ocean of Storms (sometimes known by its Latinised name, Oceanus Procellarum), the largest of the maria – indeed, vast enough that it has been granted the name of an ocean, rather than just a mere sea. The Marius Hills are interesting to lunar observers – to the point of having been a proposed Apollo landing site – because they seem to be volcanic domes.

It is that same ancient volcanic activity that formed the tunnel. The hollow tunnel is suspected to be a natural feature known as a lava tube. Lava tubes form when a flow of lava cools, creating a hard “roof,” while the lava underneath drains away, leaving empty hollow space behind. Lava tubes exist on Earth; however, this recently discovered tunnel is far larger than any known to exist on Earth.

When we look up and see the Moon, we may think that we see a lifeless, dead world, but in reality there is far more happening. As we observe the moon from our distant planet, remnants of volcanic eruptions are creating the space humans may one day inhabit. So the next time you think the moon is barren and bleak, remember all the potential it has that you cannot see.