A top secret missile bunker could be located within Southwick Tunnel, it has been claimed.
The 490 metre tunnel, opened in 1996, is an entrance point to the installation, experts believe.
Some even think the closure of the tunnel during crashes or at regular periods for "maintenance" could be a cleverly constructed lie to allow senior officials access to the bunker.
The claims will be quickly dismissed by many, but there is a growing number of people who think it is true.
Worthing-based author Graham Lelliott, who has researched bunkers and military installations across Sussex, certainly believes it could be true.
He said: "It could have been kept secret.
"Inside the tunnel, in both directions are many doors - where do they go - into electrical switchrooms - or into the bunker?
"I work nearby and the amount of times the tunnel is closed at night.
"Wouldn't this be a great chance for government personnel to change shifts or restock on food for the canteen?
"The tunnel is often closed due to an accident. The headline often reads "Two cars collide in tunnel' but maybe it should say 'Nuclear bunker shift change over."
Whether the claims are true or not, what is certain is there are at least three other military bunkers near Southwick Tunnel.
All of them are now abandoned but were likely to have been used during the 1960s and 1970s, possibly as shelters in the event of a nuclear blast.
Stuart Strong remembers seeing one in use. He said: "As a ten year old playing on the Downs at the top of Kingston Lane, in Southwick, I was conscious of a secret underground bunker there.
"I would regularly with my friends see maintenance men appear from the chamber through the large hinged hatch lid in the ground in the middle of nowhere.
"Our house backed onto farm land so I would regularly be on the Downs throughout the summer holidays and late into the warm evenings and notice this odd thing going on.
"I remember asking a man there to look inside and was told in no uncertain terms that there was nothing to see as he fastened down the heavy lid with a large padlock and a deep thud."
The MoD has refused to comment on any of its installations still in existence. But it admitted it did have bunkers in use across Sussex.
A spokesman said: "They were almost exclusively used by the Royal Observer Corps and were commonplace."
The bunkers were built to shelter from nuclear blasts and measure the damage caused.
They have been discovered in towns and villages across Sussex and are listed on the website subbrit.org.uk.