The tiny city Chiusi has probably the highest concentration of underground sites in the Tuscany. There are Etruscan water channels, tombs and cisterns, there are Christian catacombs, and there are Medieval cellars. Despite collecting some underground sites in the two major museums, there are still more sites. The city was built on top of a hill, surrounded lowlands.
The history of Chiusi is closely linked to the Etruscans, it was one of the most important cities of Etruria. The city was part of the Etruscan dodecapoli, a league of the 12 most important cities of Etruria. At this time the city was named Clevsi.
According to legend, the mythical Etruscan King Porsenna was buried here in a tomb with a golden treasure. The sarcophagus, the chariot on which it was place, the four horses which pulled, all made of massive gold. And now it gets weird: the tomb is guarded by a golden hen and five thousand golden chicks. The king built an underground labyrinth to hide the tomb and make sure it was never found. Its unclear if Porsenna actually existed, but if he existed there is most likely such a tomb, as the description resembles other Etruscan tombs. Just the parts with the gold, the hen and chicks, and the labyrinth are weird and most likely a fairy tale which was exaggerated over the millennia. Another exaggeration was obviously the huge mausoleum with a square base on which stood a series of overlapping pyramids, which reached a disproportionate height. But it's a well done fairy tale, even some archaeologists believed in its existence, and it fascinated intellectuals and artists over centuries.
The area of Chiusi was settled since the Iron Age, the oldest remains found in the city are from the 8th century BC. It became particularly important in the 6th century BC, the reign of King Porsenna. When the Romans conquered Etruria, it became more or less the center of the Roman Empire, as Rome is located inside the borders of Etruria. Their civilisation extended from The Po river to Lazio and Campania, and also included Corsica. The center was north of Rome, and Chiusi was right in the middle. Etruscan civilisation was assimilated into Roman society from the late 4th century BC as a result of the Roman–Etruscan Wars. The conquest of Etruria was completed in 265–264 BC.
Under the Romans the city was renamed Clusium, or actually it was just a different pronunciation. The Romans built other underground structures and extended the city's water system with cisterns. Pliny the Elder wrote about the city in his book Naturalis historia (XXXVI, XIX, 91-93). He describes the Labyrinth and mentions that it was part of a monument including the sepulchre of the King Porsenna. So he actually wrote down local lore and started the whole legend.