The (until 2011 secret) base of the Eternals on Mars. It lies in an eponymous geological feature in the Martian valley of Evros Vallis.
Established in the mid 1970s by Eternals Macarius and Nicolet, The Cave was only a humble to non-existent base camp at first. As secret resupply missions were flown to Mars over the years, the base had gradually grown until it enclosed the cave and became an early scientific and settler outpost on the Red Planet.
The eventual accidental discovery of The Cave by the Mars Reconnaissance Orbiter in 2011 led to the decision of the Eternals to finally break their masquerade and reveal their secrets to the world. Another resupply mission, the biggest one yet, had been deployed to the planet three years later in 2014. In addition to new equipment, resources and materials, it also brought over three new Eternals to serve as a scientific and development crew.
This additional team stayed for several years and helped with the repairs, completion and expansion of the base. Several members of both the original and second team eventually traveled back to Earth in the 2020s and were replaced at the base by another new roster of explorers. A decade later, the Eternal astronauts were also joined by regular astronaut missions launched by a conglomerate of state and private space agencies.