When golf pros and fans pull up to England’s Wentworth Club for the BMW PGA Championship, they’ll be driving in over a little-known slice of World War II history.

About 30 feet under the club’s parking lot is a sprawling bunker that was constructed by the British military and used after the war’s outbreak in 1939.

The ultraexclusive golf club, where the tournament will be played from Thursday through Sunday, sits on the Wentworth Estate in the village of Virginia Water, in Surrey, about an hour southwest of London. Now home to some of the most expensive property in the country, Wentworth was once one of many country estates requisitioned by the British military during the war.

The site was intended to be a more secure alternative to central London, especially if German bombing eventually forced evacuations from the city.

“In the war planning in the late 1930s, it was identified as a possible future seat of government,” said Alex Windscheffel, a senior lecturer in modern British history at Royal Holloway, University of London, in Surrey. “You have to remember, in the late 1930s, there’s a lot of fear about the bombers” and what they could do to cities.

But the government stayed in London after all, so Windscheffel said, Wentworth “still gets used, but I don’t think it’s ever used quite in the way that was imagined.” The Wentworth Club declined to comment for this article.

The Wentworth Estate was instead used by different military entities. According to “Secret Underground London” by Nick Catford, in the early 1940s GHQ Home Forces, the unit tasked with defending Britain in the event of a German invasion, took up residence there, with its signals regiment decamping to the bunker.

The estate later became a rear headquarters of the Supreme Headquarters Allied Expeditionary Force, the Allied command overseeing the invasion of Europe, ahead of D-Day.

The bunker consists of two parallel tunnels, each 25 feet in diameter, about 300 feet in length and divided into 11 rooms, Catford wrote for a post on Subterranea Britannica, a U.K.-based charity that documents underground sites. A smaller service tunnel runs in between the two main tunnels.

Martin Dixon, a member of Subterranea Britannica, said in an interview that the tunnels were constructed using the same cast-iron segments as the London Underground tube network, which explains why the tunnels have a look familiar to any London commuter. Each segment is still marked with “LPTB,” for the London Passenger Transport Board, the entity that oversaw the city’s public transportation at the time.

A bombproof concrete slab was placed over the bunker when it was constructed, and is now part of the club’s parking lot. “The main access to the bunker was from within the clubhouse,” Dixon said. “So up above, there would have been lots of routine activity.”

Plans in Britain’s National Archives show that the military used the clubhouse and other homes on the estate as barracks and offices, and campsites were set up around the grounds. One campsite was nestled in a cluster of tennis courts.

Parts of the estate’s golf courses were left ungroomed or had barriers added so enemy planes couldn’t land. Virginia Water Lake, a short drive away, “was drained to prevent its use as a navigation landmark by enemy pilots,” according to the Wentworth Estate site.

“It was all very hush-hush,” John Hammond, who helped build the camp, told the Surrey Herald in 1986. “You couldn’t get anywhere near the camp without a pass.” He added that the only civilian he knew of allowed inside the bunker was the electrician.

The Wentworth property was returned to its owners after the war, and the bunker was sealed. Though its structure is mostly preserved, the bunker’s walls are now covered with graffiti, and not much is left inside.

“Unfortunately when the army moved out they took everything with them, including the wooden floors,” wrote Ron Davis of the Egham-by-Runnymede Historical Society after visiting the bunker in 1990.

These days, the club rarely allows visitors to the bunker, though urban explorers seem to find their way down every so often, Windscheffel said.

“It’s potentially a very interesting historical site, but obviously it’s rather forgotten now and hidden away from the collective memory,” he said.

But Wentworth’s West course, where the tournament will be played, still holds a hint of the estate’s wartime legacy, even if it might be an apocryphal one:

The course is nicknamed Burma Road, a possible reference to a crucial supply route linking China and what is now Myanmar. As the story goes, it comes from an officer’s remark as he oversaw German prisoners clearing the overgrown West course after the war: “Let this be their Burma Road.”