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Crossing the Monastery's Loggia, from the north facade entrance towards Grimaldi Street, a series of gratings can be seen on its floor that lead to the so-called Mina de Montalvo, a passageway built by Fray Antonio San José de Pontones which, since the 18th century, connects the Monastery with the Second Casa de Oficios. From there, one would ascend to the Arch of Grimaldi Street to enter the First Casa de Oficios, from where access was gained, via another Arch located in Floridablanca Street (now non-existent), to the Real Coliseo de Carlos III. This passageway was created to avoid the extremely strong winds that occurred in the Loggia which, according to its promoter, the Count of Montalvo, would blow away the aristocracy's wigs.
Underground passage running from north to south through the Monastery's Exchange, visible through the grates in the ground of the esplanade, part of the drainage system and historical infrastructure of the Royal Site.