The Silvio Berlusconi tapes released this week have focused, not surprisingly, on lurid discussions of threesomes, condoms and staying power. But today Italy's prime minister was facing the bizarre possibility that the most explosive secret in the recordings was neither sexual nor financial, but archaeological.

In one recording posted on the website of the news magazine L'Espresso on Thursday, Berlusconi is supposedly showing the "escort" Patrizia D'Addario around his estate on Sardinia. He points out an ice-cream shop built for his guests and then draws her attention to an artificial lake with swans.

After noting that the lake is adorned by a fossilised whale, Berlusconi purportedly adds: "Underneath here, we found 30 Phoenician tombs from 300BC."

This was news to the archaeological community. And sensational news, too.

A necropolis under the estate near Porto Rotondo on the Costa Smeralda would be evidence of Phoenician settlement in an area where none were thought to have been situated. Italy's National Association of Archaeologists said it would be "of the utmost importance for the study of Phoenician expansion on the island".

Of more immediate concern was why, if such an obviously important discovery had been made during the excavation of the lake, the authorities were not notified. Government officials in nearby Olbia knew nothing about it. This is a serious matter. Failure to report an archaeological find within 24 hours is an offence in Italy punishable by up to 12 months in prison.

The opposition Democratic party, which had been looking for a way to embarrass the prime minister without getting immersed in his eventful sex life, was not slow to spot the opening. Representatives in both houses of parliament tabled questions, demanding that Berlusconi and his heritage minister give an explanation.

The prime minister has already reneged on a promise to explain his relationship with a teenager and laughed off the dissemination of the recordings. It remains to be seen whether the dead will succeed where the living have failed and force him to account for his behaviour.