The Martian underground was a de facto group and movement of Martians that lived on Mars in hiding from UNTA and metanational forces, from 2061 to 2127. They were made up of various rebellious movements that survived the First Martian Revolution. They were mainly stationed in settlements around the South pole. Many members of the underground progressively rejoined the surface society and were part of the demimonde.

The first settlement in the area was Zygote, which also became the moral center of the underground. Other important settlements were Prometheus, Christianopolis, Bogdanov Vishniac, Mauss Hyde, Gramsci, Overhangs, Dorsa Brevia, Rumi, New Vanuatu, Da Vinci. See the list of Martian settlements.

The different movements of the underground gathered in the Dorsa Brevia Conference to discuss the desired outcome of the future Second Martian Revolution they considered inevitable.