"The Block of Lights" — a colonial complex at the corner of Perú and Alsina streets that was the intellectual centre of Buenos Aires from the early colonial period through the 19th century. The block held the first Jesuit church, the first school, the national archive, and (briefly) the first printing press in Argentina.
The complex is now a cultural centre, with guided tours that include the colonial buildings and a section of the 18th-century tunnel network beneath. The tunnels are the main draw — roughly four metres below street level, brick-vaulted passages that run for over five kilometres under the old city, though only a short section is accessible to the public. Tours depart on weekend afternoons; book through the website.