The underground world had been closed for so long because it hadn’t been needed. Some sections had been turned into tour routes for those interested in history and ghost hunting. It was never thought that the space would ever be required again.
Then the outbreak happened. It was pushed off as a simple disease before levelling up to a world sweeping plague. Millions of people died but they didn’t stay in their graves.
The government repaired, improved and kitted out the underground cities, factories and farms once more. People moved in, coming not just from the country above but from others as well. Builders had to construct new sections and everyone had to pull together to survive.
The world above become ravaged by war and death, disease ran wild and people returned from the other side, their bodies reanimated, their hearts and brains nonfunctional.
Anybody who died in the underground world was thrown out the steel doors and just left there. Burials, prayers and rites no longer had a place and the families could only hope their loved ones stayed dead.