Like many present-day green spaces, Chicago’s verdant Lincoln Park had a past life as a cemetery. Ambling through it today, though, you’d never know it—unless you happened across the Couch Tomb.

The stone mausoleum is the most visible record of the park’s history as the Chicago City Cemetery, and it’s shrouded in mystery. A few details are set in stone, but others are much gauzier.

Here’s what’s certain: The crypt was commissioned in the mid-1800s by Ira Couch, a businessman in the game of real estate and hotels, who died in 1857. At the end of the 1860s, when the burial ground was slated to become park land, relatives of the deceased interred there were responsible for transplanting the graves’ contents elsewhere. Remains were often reinterred in Graceland or Rosehill cemeteries, which were slightly north of the city in areas that were then considered rural. The Couch tomb stayed put, though, and it’s not quite clear why.

Some reports, both historic and contemporary, suggest that hauling it elsewhere would have been a tremendous hassle. It took a band of horses to drag the hefty marble structure across town once, and maybe the Couch family didn’t want to do it a second time.

It’s also said to be one of the few still-standing structures that endured in the path of the fire that licked through in 1871, and a few decades later, visitors were already considering the tomb to be a wondrous oddity: “The vault, surrounded by great trees, stands there in the park, a subject of great curiosity to visitors,” the Chicago Tribune reported in 1892. A century hence, in the 1980s and ‘90s, the vault was swallowed by thick tangles of greenery, and the bushes earned a reputation as a hookup site.

It’s withstood time, disaster, and neglect and clung to its secrets all the while. In recent years, some scholars have set out to excavate the history, but major uncertainties remain. Chief among them: Who are the occupants, and how many are there? No one is sure. (Some estimates say there’s probably only one body inside, while others go as high as 13.) Centuries after the vault was built questions linger, like ghosts.