JERUSALEM – Underneath the crowded alleys and holy sites of old Jerusalem, hundreds of people are snaking at any given moment through tunnels, vaulted medieval chambers and Roman sewers in a rapidly expanding subterranean city invisible from the streets above.

At street level, the walled Old City is an energetic and fractious enclave with a physical landscape that is predominantly Islamic and a population that is mainly Arab.

Underground Jerusalem is different: Here the noise recedes, the fierce Middle Eastern sun disappears, and light comes from fluorescent bulbs. There is a smell of earth and mildew, and the geography recalls a Jewish city that existed 2,000 years ago.