One of the most interesting discoveries when exploring the underground is the medieval cellar in Biskupská Street on Petrov. It is hidden beneath the triangular area of the lawn, where in the 14th century at the stairs stood the pub ‘U veliké dýmky’ (At the Big Pipe), known colloquially as ‘U fajfky’. The pub included stone cellars, which served the needs of the inn. According to the 1867 advertising leaflet Moravské orlice (Moravian Eagle), the ‘good old pub U velké dýmky’ in Petrov with its ‘well-poured lager’ was very popular. Fresh homemade sausages were served on Saturday. The regular guests of the pub were ‘honourable master craftsmen’, who stopped here especially on Sunday after mass or after blessings for a morning or evening draught. All of the buildings except the cellars were torn down in 1888.

The cellars of the former pub were discovered during an underground exploration in the early 1990s. They were carved in the rock, with brick walls added later. The cellars have two wells that are still active. That is why this cellar is constantly flooded with groundwater; if not drained, it creates a natural water reservoir. For this reason, it was necessary to use an inflatable boat during the exploration and to float under the vault. Lamplight revealed karst decorations. This is the origin of the legend about a lake under Petrov.

During the reconnaissance and repair of the cellar, both wells were cleaned; the masonry and floor, almost destroyed by long-term exposure to water, were repaired; and the staircase by the entrance shaft was restored. An unresolved problem of this space, however, remains the rising water levels of both wells, which have to be pumped out regularly. Access to the cellars is now only possible through a steel hatch located in the grassy area.