Police arrested an “urban explorer” spotted in an abandoned Manhattan subway tunnel after officers found him armed with a loaded pistol, cops said Thursday. Officers caught 19-year-old Jaiden Johnson in an off-limits section of the No. 4 line near the Brooklyn Bridge-City Hall stop in the Financial District around 4:25 p.m. Saturday. After searching Johnson, police discovered he was armed with a Cobra semi-automatic pistol, the NYPD tweeted following the arrest. “An urban explorer found himself in hot water recently, when he was discovered trespassing in an off-limits underground location,” read a tweet from the NYPD Chief of Transit account. “He made quite the stink when officers handcuffed him, and we soon learned why — he was carrying this loaded gun.” The Brooklyn Bridge-City Hall Station is located near the City Hall Station, an abandoned stop along the IRT line and a popular destination for transit enthusiasts. Decommissioned in 1945 for lack of use, the City Hall stop is nevertheless admired for its Romanesque Revival architecture characterized by vaulted ceilings, Guastavino tilework, skylights and brass chandeliers. The New York Transit Museum provides sanctioned tours of the City Hall Station three times a year, offering the only legal way to step foot inside the abandoned stop. Southbound No. 6 train straphangers willing to wait at the Brooklyn Bridge-City Hall Station, however, can catch a glimpse of the old City Hall stop as the train switches over to the northbound tracks.