A series of mysterious rooms have been uncovered for the first time in 250 years at Blenheim Palace, the birthplace of Winston Churchill in West Oxfordshire.

The rooms are part of a “lost” ground floor within the site’s Grand Bridge. Flooded when water levels were raised to create lakes in the 18th century, the rooms have been rediscovered thanks to a massive dredging effort to remove silt from the lakes. As a result, water has been drained away from the “lost” floor.

Experts exploring the rooms have found graffiti dating back to the 1760s and a puzzling blocked-up tunnel. Sunken boats, which are believed to date back to the 1950s, when they were used for reed cutting, have also been exposed.

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“The Grand Bridge is one of the most intriguing and fascinating buildings at Blenheim,” said Blenheim’s Head of Estates, Roy Cox, in a statement. “We’re currently undertaking a full internal 3D survey as part of a major restoration project.”

The hidden rooms have been revealed after 250 years underwater. (©Blenheim Palace)

“It has already revealed a large number of rooms and passageways, some containing original plasterwork, stairways and potentially cooking ranges,” he added.

Created by the architect and playwright John Vanbrugh in 1708, the ornate bridge was designed to be a “habitable viaduct” and is said to contain over 30 rooms.

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Although there is no evidence that the bridge was ever lived in, officials note that some rooms have fireplaces and chimneys. A large chamber within the imposing structure is also plastered and fitted with an arch, resembling a theater, they explain.

Sunken boats were revealed as the waters drained away. (©Blenheim Palace)

The residence of the Dukes of Marlborough, Blenheim Palace is a UNESCO World Heritage Site. The view across the lake over the Grand Bridge to the 18th-century Baroque palace was described as the finest in England by Winston Churchill’s father, Lord Randolph Churchill.

Grafitti dating back to the 1760s was found in the rooms. (©Blenheim Palace)

In July President Trump joined British Prime Minister Theresa May for a black-tie dinner with business leaders at Blenheim Palace. The four-day trip was Trump's first to Great Britain as president.

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The discovery is the latest fascinating historical find in the U.K. in recent times. A mysterious painting hidden for centuries at the Tower of London may be a self-portrait of the famous adventurer Sir Walter Raleigh, officials recently revealed.

The rooms are part of a “lost” ground floor within the site’s Grand Bridge. (©Blenheim Palace)

Earlier this year, an extremely rare medieval Bible was returned to the cathedral it was taken from 500 years ago. The Bible was written in the late 13th century, according to Canterbury Cathedral in the U.K., which is now in possession of the item.

Last year, archaeologists in London uncovered the remains of Greenwich Palace, the birthplace of Henry VIII and Elizabeth I.

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The Grand Bridge at Blenheim Palace, Oxfordshire, England. (©Blenheim Palace) (Education Images/UIG via Getty Images)

The palace once stood on the site now occupied by the Old Royal Naval College in Greenwich, southeast London. Two rooms from the Tudor-era palace were found when the ground beneath the Old Royal Naval College’s Painted Hall was being prepared for a new visitor center.

The Associated Press contributed to this article.

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