Beneath a 19th-century colonial house in San Telmo, a network of tunnels dating from the 18th century runs under the old city. El Zanjón is a private archaeological site and restored museum — the tunnels follow the course of the Zanjón de Granados creek that was channelled underground as the city grew, and they pass through and under structures from four centuries of Buenos Aires history.
Guided tours are the only way to see it, and they are good: the guides know the archaeology well and can explain what each layer of brickwork represents in terms of the city's development. The site was used variously as a water channel, a drainage system, a wine cellar, and a cistern over the centuries. Allow 90 minutes; book ahead.