The Priory has always been regarded as a place of secrets and mysteries. Various legends about it have appeared, one after another. One of the most striking of these concerns the existence of a secret underground passage, supposedly connecting the Priory palace with the Gatchina imperial palace. There existed a popular belief that Paul I ordered his chamber, in the first tier of the Guard Tower, to be connected with the chamber of the prior of the Maltese order in the Priory palace.
During the strengthening of the foundations the restorers did, indeed, come unexpectedly upon an underground passage. One of the bolder spirits among them went along it for about 100 metres, but to continue further without special equipment was too risky. At the start of the passage its ceiling was high enough for a man to stand upright, but became gradually lower. The passage was lined with stone. The exact purpose of this mysterious tunnel was not clear. In the opinion of the restorers the passage runs beneath several reservoirs and is a part of old Gatchina's underground lines