Hidden under the Millennium BCP bank building are layers of ruins dating from the Iron Age, discovered on a 1991 parking-lot dig. Fascinating archaeologist-led tours, run by Fundacão Millennium (booking ahead year-round is highly advisable), descend into the depths in English (departing on the hour). The extremely well-done site is now rightfully a National Monument.
You'll visit a small museum of artefacts found on premises before heading down into the web of tunnels, the majority of which are believed to be the remnants of a Roman sardine factory (and its owner's home) dating from the 1st century AD. It's worth noting that archaeologists had to remove Medieval and Islamic ruins (among others) to reach these startling structures. Highlights include Lisbon's only visible Roman mosaic, dating to the 3rd century; numerous baths and fish-preservation tanks and a Visigoth burial site with a remarkably preserved 30-year-old male skeleton. The entire 850-sq-metre site is notably well-maintained and is easily one of the city's most fascinating (and free) attractions.