In Rome they have been working since 15 years to the construction of the third line of the metro (metro C). Aside from some corruption and various delays, the line isn't completed yet. And while they dig they keep finding exceptional things...like the one described in this article (it is in italian). It is saying that during the construction of the metro they found 80 metres (roughly 250 ft) of the original walls built at the end of III century AD. These walls were built to protect Rome from possible invasions of enemies. What they discovered is exceptional because for centuries they thought that part of the walls weren't existing anymore...but they was just buried underground very close to the Basilica of St. John in the Lateran. This part of the walls still preserves also some traces of frescoes.