A network of underground passages running beneath the historic centre, built over several centuries for drainage, military movement, and escape routes. The tunnels were long regarded as urban legend — locals whispered about them but no one could verify their extent. Systematic excavations beginning in the early 2000s confirmed the network's existence and opened sections to the public.
The accessible sections have been cleaned, lit, and fitted with walkways; guided tours run from several entry points in the centro histórico. The tunnels date from different periods — some sections appear to be 16th century, others later — and their full extent remains incompletely mapped. Check with the tourist office at the Zócalo for current tour access points, as these change. Tours typically take 45–60 minutes and are available in Spanish; some English-language guides can be arranged.