Mel Gibson and his soon to be ex wife, Robyn Moore Gibson, are about to embark what is rumored to be the most expensive divorce in history. Mel is rumored to be worth nearly $1 billion dollars according People. The Gibsons had not signed any kind of prenup from their 1980 marriage. Without a prenuptial agreement the state of California legally enforces a 50/50 split of the entire estate. The wife walks away with half. Which means Robyn is could walk away from their marriage with $500 million.

They have indeed been separated since August 26, 2006 according to Mel’s official filed response. This was about a month after his drunken DUI arrest and anti-Semitic bumbling. So what was the final straw that has officially ended their marriage? It is rumored to be his mistress Oksana Kolesnikova.

[Oksana Kolesnikova Playing Piano Video]

Alleged photos of Mel and Oksana Kolesnikova surfaced last month (see those by clicking here) of the two vacationing in Costa Rica. Oksana and Gibson were embracing and kissing on the beach reported a few witnesses. Meanwhile Mel’s wife was no where to be seen. However, several members of the Gibson family were also seen with the two. Mel and his mistress were first seen in Boston last year. Rumors that she spent ample time with him on the set of “Edge of Darkness.” She was even seen coming from his private trailer several times.

Oksana, a Russian pianist, denies any kind of ties with Mel.

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Images Via: Radar, Christopher Ameruoso