Surely one of the things Quantum Theatre's audiences appreciate most is the urban-egg-hunt aspect of following the nomadic company's productions to offbeat spaces around town.

Quantum is of course the company that by the design of founder and artistic director Karla Boos stages only site-specific work, invariably in nontraditional performances spaces. Most notable, perhaps, there have been parks, a cemetery, a drained and long-unused public swimming pool, and a numberless succession of cavernous, vacated warehouses. Quantum's been at it for 20 years.

The latest venue is the old East Liberty Y. Though located right in the heart of that rejuvenating neighborhood -- and right across from the grand East Liberty Presbyterian Church, next to the Carnegie Library and nearby a growing roster of good restaurants -- the century-old building has stood empty for years.

Lots of dust and peeled paint aside, it's still a pretty solid old pile. Just a few years ago, in fact, there was a scheme to turn it into luxury condos.

But if it's got to be empty, it's hard to imagine a better temporary use than Quantum's production of Maria de Buenos Aires, a "tango operita" by Astor Piazzolla. The building's aura of ruined neoclassicality seems ideal for the opera's surreal narrative about a prostitute's hard life, death, sojourn in the afterlife and rebirth.

The story is on the mythic side and a bit difficult to follow. (It's entirely sung and spoken in Spanish, only some of which is translated vocally or by video projections.) But the music, played by a live orchestra, is gorgeous, and what dialogue and lyrics we do catch are often evocative. ("The hearse was pulled by 12 Judases and driven by a small drunken Christ.")

Quantum has even turned the performance space -- which looks like it might have been the Y's small auditorium or banquet hall -- into a cabaret, the multi-runway stage surrounded by small café tables bolted into the floor.

With the music and the darkly redemptive action on stage (including dancing choreographed and performed by Attack Theatre), it's a different little world right in the middle of the world you thought you knew.

Maria de Buenos Aires continues with four more performances through Sun., April 17 (including two the evening of Sat., April 16). www.quantumtheatre.org