Survival Rules for the Apocalypse: Don't Ask Where the Source of Heat Comes From

[The end of the natural disaster, not a cool article, no system, no superpowers] [Survival + Humanity] This city is built 700 meters underground and is the last refuge of mankind. Officially, it is called "Wenchao", which means a warm nest. But people who live here call it "the crematorium." Because the people who live here are destined to spend their lives here. The outside world was swallowed up by an unprecedented catastrophe - the sun disappeared, the temperature plummeted, agriculture collapsed, smoke and dust flew everywhere, and civilization leaked from the fingers like sand. The survivors huddled underground, relying on a little warmth from the ground that came from unknown sources to survive the sunless days one after another. There are no heroes or saviors here, just ordinary people struggling to survive. Rations are getting smaller and smaller, rules are getting stricter, and the number of lunatics and thugs is increasing day by day. There are even rumors that someone heard a sound behind the wall late at night - an ancient, heavy sound, like a huge heart, beating slowly underground seven hundred meters deep. Yes, the city is keeping them alive. But what is it that sustains this city?