Experts at the Galleries of Justice in Nottingham think they may have found the site of the old prison chapel.
While researching their latest exhibition, staff found old prison reports dating back to 1838.
Reports revealed clues to the potential location of the secret chapel, saying it was like a "parlour... accessible by a small narrow staircase".
After studying maps and old floor plans, the curator realised the chapel was part of the medieval cave system.
Blackened charred stone surrounding a makeshift altar, and a cross on the wall, all indicate that the chapel was in a cave and dates back to the early 1700s, pre-dating the jail.
The Galleries of Justice once housed Nottingham's court houses, police station and jail.
Used as a venue for trials since the 13th Century, the Victorian prison closed in the 1870s and the police and courts moved out in 1980s.