31 Days of Halloween: On Atlas Obscura this month, every day is Halloween. Stop by the blog every day this month for true tales of the unquiet dead. Come for the severed heads, stay for the book bound in human skin. Every story is true, and each one is a real place you can visit. We dare you.

Today we find ourselves wandering the largest catacomb system in the world. Try not to get lost.

On January 1st 2005, a young Ukranian teenage girl named Masha put on her make-up, packed up some champagne, and headed out for her New Years Eve nights party. She was headed into the Odessa catacombs, the 2,500 kilometers of dark rocky tunnels which run underneath the city.

The catacombs were once the preferred hideout of rebels, criminals, and eccentrics. During WWII Ukrainian rebel groups hid within the labyrinthine catacombs and today there is an entire Ukrainian subculture of catacomb explorers with dozens of semiprofessional groups, often quite competitive, exploring the catacombs. They go on multi-day underground treks, known as expeditions, to document and map the system.

The 2005 New Years eve party was great. Everyone was having a good time, drinking and having fun. But at some point in the night Masha decided to explore the catacombs. This was a mistake. She quickly became separated from her party, and unable to find her way back. Nor were her friends able to locate her. A massive search effort was launched but they weren’t able to find her among the 2,500 kilometers of often unmapped tunnels.

It took a full two years before the police were able to locate her body and retrieve it from the catacombs. Her body had been partially mummified by the cold cool air. Based on where she was found it is believed Masha spent at least three days wandering in the freezing cold and pitch black before dying of dehydration.