In the 1967 episode "The Devil in the Dark" the Enterprise visits the mining colony on the planet Janus VI, or better, it is located inside in the planet, as the mine is a large underground facility. Soon the crew encounters an unknown creature. The episode later reveals that the alien life-form, named "Horta", comes from within the planet, where suitable conditions for life might exist, and feeds on rocks.
"It is not life as we know or understand it!"
In October 2018 the discovery of cyanobacteria living 600 meters underground was announced by a research team from Spain, Germany and the United States. The rivers in the Rio Tinto region in Spain is poisoned by iron and sulfur minerals, also a legacy of mining operations of the past. Because of its apparent inhospitality of life as we know it, this region is studied by astrobiologist, hoping here to understand how life adapts to harsh environments like found on other planets. In a 613-meter deep borehole, the researchers found cyanobacteria living in cracks and crevices of the rocks. Cyanobacteria are able to do photosynthesis, so it was believed that they need sunlight to survive. The cyanobacteria discovered in the borehole apparently feed on chemical reactions involving hydrogen gas, found in air pockets inside the rocks.
Viable cyanobacterial cells (red fluorescent signals) attached to rock fragments. Credit: Puente-Sánchez et al. 2018.
Living bacteria (not depending on sunlight) were found in boreholes in 5,278 meters depth. Living underground has some advantages. Lifeforms on a planet without atmosphere are protected by the ground from harmful radiations. Even on Earth, as some research suggests, life for a long time existed in the underground. Much of the life that exists beneath the surface is of ancient origins. This realm is inhabited by bacteria, existing already 2 billion years ago. Most likely during most of life's history, for almost 1.5 billion years, organisms living beneath the Earth’s surface outweighed all other life on the planet combined. Exobiologist are especially interested in this observation, as it could vastly expand the habitable zones found in the universe. Even on superficially desolate planets some life could exist, it would just hard to spot it, as it would be hiding deep within the planet.
"They're eggs, aren't they?"
