On a warm spring day, nine people were gathered inside a cold crypt beneath the Basilica of St. Patrick’s Old Cathedral on Mulberry Street . “This is the size of a standard New York apartment,” the tour guide, Brandon Duncan, said to the Texas and Austrian tourists standing in the spacious room — the final resting place of a long-forgotten Civil War general.

Overhead were polished Guastavino tiles, original Edison light fixtures and space for eight more bodies, marble lids propped up and ready. “It’s sort of like a hostel situation,” Mr. Duncan said, glancing up at the empty bunks. The general, Thomas Eckert, left varying degrees of inheritance to his children, who wound up bickering and perhaps decided not to spend eternity together, Mr. Duncan explained.

Eckert’s vault doesn’t get family visitors anymore, but provides the grand finale of a 90-minute candlelight catacombs tour, opening up one of New York’s most secret spaces to the public. St. Patrick’s Old Cathedral — often referred to as the other St. Patrick’s Cathedral — recently developed this and other programming to help pay for the church’s upkeep and its historic outdoor cemetery, the only active Roman Catholic cemetery in Manhattan.

“We have a long way to go in making the grounds the way we want them,” Msgr. Donald Sakano, who took charge of the NoLIta parish a decade ago, said. “The gravestones are deteriorating as we speak, the soft stone losing inscriptions.”