A LANCASTER bomber flying over the Hampshire countryside dropped one of the revolutionary new Grand Slam devices invented by the legendary Barnes Wallis.

The resulting explosion, which caused the earth to shake for miles around, created a massive crater more than 70 feet deep and 130 feet across.

Now experts are using new techniques to unearth the remains of the Ashley Walk bombing range in the New Forest, where the Grand Slam was dropped in 1945.

They hope to solve the mystery surrounding rumours that smaller bombs were buried on the range when the war ended.

Ashley Walk occupied a 5,000-acre site between Eyeworth Wood and the B3078 Bramshaw to Godshill road.

Wallis was among those present when his latest invention was tested on the range.

The following day a Grand Slam was used to destroy a viaduct that carried the main railway line from northern Germany to the Ruhr.

Similar “earthquake bombs” were deployed against a range of other targets in Nazi-occupied Europe including bridges, submarine pens and coastal gun batteries.

The 22,000lbs devices were designed to penetrate deep underground before causing an explosion that wrecked the target’s foundations.

Dummy targets built at Ashley Walk included a large concrete structure dubbed the Sub Pen because of its similarity to German U-boat facilities.

After the war it was buried and has remained largely hidden from view for more than 70 years.

But archaeologists led by the National Park Authority (NPA) are using ground penetrating radar and electrical imaging to peer beneath the earth.

An NPA spokesman said: “They plan to discover what remains of the ‘Sub Pen’, confirm whether the chambers inside still exist and whether the structure needs future conservation.

“Their non-intrusive methods could also shed light on rumours of buried bombs or airplane parts inside the structure.

“Some of the techniques will be familiar to Time Team viewers.”

The groundbreaking scheme is part of the New Forest Remembers World War II Project, funded by the Heritage Lottery.

As reported in the Daily Echo, the NPA is also using an airborne light detection and ranging system (Lidar) to uncover other long lost structures in the Forest.