All of Buenos Aires' history is packed into this unusual house. The street it's on was once a small river—the zanjón, or gorge, of the property's name—where the first, unsuccessful attempt to found Buenos Aires took place in 1536. When the property's current owner decided to develop what was then a run-down conventillo, he began to discover all sorts of things beneath it: pottery and cutlery, the foundations of past constructions, and a 500-foot network of tunnels that has taken over 20 years to excavate. These were once used to channel water, but like the street itself, they were sealed after San Telmo's yellow-fever outbreaks. With the help of historians and architects, they've now been painstakingly restored, and the entire site has been transformed into a private museum, where the only exhibit is the redbrick building itself. Excellent hour-long guided tours in English and Spanish take you through low-lighted sections of the tunnels. The history lesson then continues
aboveground, where you can see the surviving wall of a construction from 1740, the 19th-century mansion built around it, and traces of the conventillo it became. Expect few visitors and plenty of atmosphere on weekdays; cheaper, shorter tours on Sunday draw far more people. If you want to spend even more time here, you can rent the whole place (including an adjacent building reached via the tunnels) for functions.