Last updated at 18:47 26 June 2007

Most of us finish a day's gardening with a bad back and grubby fingernails. But Andy Lewis's labours were far more rewarding.

He returned from his toils having discovered what could turn out to be a sizeable addition to the value of his three-bedroom terrace.

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In the overgrown patch of shrubs at the end of his plot he found a manhole, beneath which was a rusty ladder descending 12ft into the darkness, then opening out in to an underground bunker measuring an astonishing 50ft by 30ft.

"I was absolutely amazed," he said. "It was completely mad. There was a big empty room with two pillars in the middle."

Even more exciting were some of the items he found down there, including a tiny doll, a toothbrush, food tins, part of a child's shoe and a number of coins dating back to the 1860s.

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The 1950s house, in Old Sarum, near Salisbury, Wiltshire, was formerly owned by the Ministry of Defence and was built on the site of a disused airfield. Mr Lewis, 36, an employment adviser, and his partner Fiona Marks, 27, have been told by a local historian that the bunker may have been used during the two world wars as storage for water, fuel or gas.

Another suggestion is that it was built as a base for a Home Guard resistance force had the Nazis invaded Britain.

As for the items Mr Lewis found, the only explanation that has emerged is that they belonged to children who may have used the bunker to play in after the house was built.

The couple were clearing a section of the garden where they were planning to build a play area for their children - Megan, six, Ellie, two and 14-month-old Alfred - when Mr Lewis's garden fork struck metal and the manhole was exposed.

After prising it open, he found the ladder too inviting to resist and began the descent.

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"We were carrying out some renovation work on a huge overgrown patch when we made the discovery," he said. "As we moved back a pile of soil, I found this manhole cover in the earth.

"We only moved into the house a year ago and there was nothing on the searches to say there was a sewer pipe so we thought we'd have a look.

"After prising the lid off with a spade, I saw some rusty rungs leading down into a deep hole.

"I got a torch and went down with Fiona following.

"The floor was covered with rubble and old pipes but as we looked closer we found a tiny doll, a rusty toy car that was stuck to the floor, some rusty spoons and three food tins. The food tins just disintegrated in our hands."

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Miss Marks continued: "Andy has spoken to a local historian who said our house is built on an old airfield.

"He thinks the room was used to store fuel, water or gas during the wars. He believes the items had been left by children who had played down there in the 1960s.

"I think it's amazing but we just don't know what to do next. I want to turn the room into an underground swimming pool or put a hot tub and sauna down there. But Andy wants to have a games room or a pub.

"All our neighbours have been digging up their gardens to see if they have a room too.

"The man next door found a manhole cover but the hole it covered was just big enough for me to stand in.

"It would be fantastic to think there could be an entire secret underground city down there."