This seemingly indestructible pile of bricks has been here for 2,000 years and will probably remain for another 2,000. Like the larger tomb of Hadrian across the river, this was once a circular marble-covered affair with tall cypresses, symmetrical groupings of Egyptian obelisks, and some of Europe's most spectacular ornamentation. Many of the 1st-century emperors had their ashes deposited in golden urns inside, and it was probably because of this crowding that Hadrian decided to construct an entirely new tomb (the Castel Sant'Angelo) for himself in another part of Rome. The imperial remains stayed intact here until the 5th century, when invading barbarians smashed the bronze gates and stole the golden urns, emptying the ashes on the ground outside. You can't enter, but you can walk along the four streets encircling it.