Basilica of San Clemente

Three churches one above the other, and you really do go down. At street level the twelfth-century basilica, with the apse mosaic of the tree of the cross. Below it, the fourth-century early Christian basilica, with medieval frescoes. Lower still, some fifteen metres down, a first-century Roman building and a Mithraeum, the altar of the god Mithras still in place and a watercourse running in the dark.

Did you know: On a wall of the lower church there is an eleventh-century inscription regarded as one of the earliest written traces of Italian rather than Latin. It is a string of insults, spoken by men dragging a saint away: the language we speak begins, in writing, with a swear word.

✦ The legend: Clement, the fourth pope, was condemned to hard labour in Crimea and then thrown into the sea with an anchor round his neck. Legend tells that the sea withdrew for a week each year, uncovering a marble chapel built by angels with his body inside. That is why, in the basilica's mosaics and paintings, St Clement is shown beside an anchor.