One of the most interesting discoveries during the underground exploration is the medieval cellar in Biskupská Street in Petrov. It is hidden under a triangular area of lawn, where in the middle of the 14th century the chapter pub "U veliké dýmky" (At the Great Pipe), popularly called "U fajfky" (At the Pipe), stood in the place by the stairs. The pub also included rock cellars which served the needs of the inn and, according to an advertisement in the Moravská Orlice newspaper in 1867, the 'old-fashioned pub U velké dýmky' in Petrov with its 'well-aged lager' was one of the most popular. On Saturdays, fresh homemade liverwurst was served. Regular visitors to the pub were the 'honest master craftsmen', who would stop in especially on Sundays after mass or after the blessing for a morning or evening sip. All the buildings, except the cellars, were demolished in 1888.
The cellars of the former pub were discovered during underground exploration in the early 1990s. They are carved in the rock with later brick linings. The cellars also include two wells which are still active. Therefore, this cellar is constantly flooded with groundwater, which, if not drained, creates a natural water reservoir. It was therefore necessary to use an inflatable boat to explore the vault and to pass under the karst decorations by torchlight. This is probably the origin of the legend of the "lake under Petrov".
During the exploration and repair of the cellar, both wells were cleaned, the masonry and the floor, which had been almost destroyed by the long-lasting effects of water, were cleaned and the staircase, which ended with an entrance shaft, was restored. However, the rising water level of the two wells still remains an unsolved problem in this area, which must be drained regularly for the time being. Access to the cellars is now only through a steel hatch in the grassed area.