Green’s Dictionary of Slang

catacomb n.

(N.Z. gay) an underground public lavatory used for soliciting.

[NZ]W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 63: As underground toilets began to emerge [...] in the bog trade they were generically termed catacombs or grottos.