The crusher is believed to have fallen through an underground cavity

The men had to unhook themselves from scaffolding before they could flee from the falling machinery.

Health and Safety officials are at the scene of the old Mothercare site opposite Boots to investigate the incident.

No-one was hurt at the site which is part of a multi-million pound redevelopment of the city centre.

If we had not been looking we would have been killed Scott Brierley

Demolition worker Scott Brierley said: "We were unloading some gas bottles when I realised something was not right.

"You just have a gut feeling when something is that close.

"If we had not been looking we would have been killed. There would have been no escape."

Fellow worker Joby Trow said: "We had our harnesses connected so we could not run.

"We did panic, but we disconnected our harnesses as quickly as possible."

The base of the crushing machine apparently broke through into an undetected void below ground.

�170m revamp

Berkhamsted-based demolition contractor Davis & Samson was unavailable for comment.

Demolition work started last month as part of the �170m revamp of Drake Circus.

It is hoped the new shopping centre, a state-of-the-art 60,000 square metre development featuring 45 shops, cafes and restaurants, will be ready to open in autumn 2006.