First of all, despite the name and the address, the entrance is NOT on Rua dos Correiros. I went twice, and the door was locked at both times. There is a sign, so I know I was at the right place and at the right time, I just thought they were just not abiding by the opening hours, but finally found out that the real entrance is on the other side of the building, on Rua Augusta 69.
When we finally walked in, the receptionist told us that visit is guided tour only, so we signed up for the one coming up in half an hour, and easily killed time in nearby shops.
Background story: Millennium Bank bought this property in the 1990s, but during renovation, they unearthed archaeological structures from the past as far back as Iron Age.
If you love history and archaeology, this place is worth a visit, as the guide will walk you around to show you the various structures, from a fireplace in the Iron Age, the mosaic during the Roman Empire (when the city was called Olisipo), pottery from the Medieval period, misshapened glass from the 1755 Earthquake and so on. Interesting to see the layers of human activity throughout the ages. There are also displays of excavated objects: pottery, coins, glasses and more. It's not a lot of see, don't expect a museum, but it's free, and nice and cool underground on a hot day! read more