The historic well is 58.44 m deep and has a cylindrical shape with a diameter of 13 m. Around the opening there are two independent stair ramps, each with 248 steps. They are illuminated by 70 small windows placed in rows along the inner wall of the well. They are wide enough for draft animals carrying vessels of water to pass over them.
In 1527, Rome was sacked by the Spanish-German army. To ensure that residents had access to water in the event of another siege, the Pope ordered a well to be built. It uses an underground watercourse that flows between the rocks on which the city stands. The construction of the well lasted ten years and was completed in 1537 during the pontificate of another pope, Paul III.
The name of the well refers to its depth and medieval legends about the Irish cave called St. Patrick's Day.