One of the biggest mysteries in global finance has just been disclosed: Saudi Arabia owes United States a whooping some of $117 billion.

The Treasury Department announced on Monday, ending four decades of keeping the figure secret, that Saudi Arabia stockpiled $116.8 billion of U.S. Treasuries as of March.

Though well behind the $1 trillion-plus owned by China and Japan each, this discovery makes Saudi Arabia the 13th largest foreign holder of U.S. debt. The Saudi figure was first reported by Bloomberg News based on a Freedom of Information Act request.

The Treasury Department kept Saudi Arabia's precise holdings secret since the 1970s. Saudi's holdings were lumped together with that of other oil exporting nations, including Venezuela and Iraq.

However, this was brought to an end as the treasury department disclosed precise holdings by specific countries that were previously grouped together.

A treasury official told newsmen that this move became necessary following a review aimed at trying to provide more ‘comprehensive and transparent’ data.

There is a twist to the latest development as it is possible that Saudi Arabia owns even more U.S. debt than what was revealed on Monday. That's because Saudi Arabia's central bank listed owning $587 billion of foreign reserves as of March. Typically, central banks park the majority of their foreign reserves in U.S. Treasuries – the numbers don't really add up.

According to reports, the mystery had taken on greater significance in recent months. Since the end of 2014 the Saudis have burned through more than $130 billion of foreign-exchange reserves to help cope with the crash in oil prices.

The treasury department said Saudi Arabia's U.S. debt holdings of $116.8 billion are down from $123.6 billion in January.

The new treasury report also revealed that the Cayman Islands, with about 60,000 people, owned $265 billion of U.S. Treasuries as of March. That's the third-highest sum in the world and a reflection of the nation's status as a major tax haven. The Cayman Islands does not have a corporate tax, encouraging multinational companies to store vast sums of money there to avoid taxes.

Bermuda is another popular tax haven; sitting on $63 billion of U.S. debt. Previously both the Cayman Islands and Bermuda were lumped together in a group of Caribbean banking centre.