|Ruby Rubacuori Photo: Rex Features|
The existence of the cave at the former Italian prime minister's Villa Certosa estate had been rumoured for years but the first photographs of it emerged on Friday.
They were obtained by Antonello Zappadu, an Italian photographer who made his name in 2009 by taking long-lens pictures of scantily-clad showgirls at Mr Berlusconi's residence.
The cave, which could have come straight out of a James Bond film, features a swimming pool, its floor decorated with a mosaic of Poseidon, the god of the sea, holding a trident.
It is connected to a short tunnel and a gate which reportedly leads straight to the sea.
The cave is connected to the rest of the villa by stairs and a passageway, which are illuminated by discreetly placed lights set into the floor.
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The underground complex is believed to have been built in 2004 but has never been seen publicly.
"It was the secret services who asked me to carry out the work, after I had received 38 death threats," Mr Berlusconi explained at the time.
He said he had sought advice on the design and construction from Pietro Lunardi, a cabinet colleague who was then the minister for infrastructure and insisted that no planning regulations had been broken.
Mr Zappadu, who now lives in Colombia, said on his blog that he had obtained the images from an anonymous source.
The semi-submerged cave is not the only flamboyant feature at Villa Certosa, a 150-acre estate where Mr Berlusconi has hosted the likes of Tony Blair and Vladimir Putin.
He once built an artificial volcano in the grounds of the villa to entertain guests at one of his parties.
A week ago he injured his shoulder and wrist after taking a tumble at the villa, although his personal doctor said the damage was not serious.
Mr Berlusconi, who turns 76 this month, stepped down in November when Italy became immersed in the eurozone crisis and after his reputation was damaged by a series of sex scandals, most notably involving allegations that he paid for sex with an under age prostitute at "bunga bunga" parties at his mansion in Milan.
Karima El Mahroug, better known by her stage name Ruby the Heart Stealer, and Mr Berlusconi both deny the charges. The ongoing trial is not expected to finish until the autumn with dozens of witnesses due to testify including George Clooney and Cristiano Ronaldo.
Mr Berlusconi and his allies have dropped heavy hints that he is planning to contest Italy's next elections, which are expected in April when Mario Monti, the caretaker prime minister, is expected to step down.
It would be his fourth stint as prime minister, although his People of Freedom party currently only has around 20 per cent of the vote.
|Vladimir Putin surveys the scene from the back of a boat in Karelia, north-west|
Russia. A new report says the Russian president has overseen a phenomenal
expansion in the awarding of presidential perks. Photograph: RIA Novosti/Reuters