Neo: Zion?
Tank: If the war was over tomorrow, Zion’s where the party would be.
Neo: It’s a city?
Tank: The last human city. The only place we have left.
Neo: Where is it?
Tank: Deep underground, near the Earth’s core, where it’s still warm. You live long enough, you might even see it.

Zion is a (planned) self-sustainable base/town, one kilometer underground.

Currently, the only entrance is through the dzwdzhole. A fast-travel shaft is planned.