A strange crater appeared in Potters Fields Park on Saturday as part of the launch of Vauxhall's new Insignia car range.
The large object had 'landed' on the grass next to City Hall and was emitting smoke and a strange noise.
Men in dark glasses were guarding the crash site and refusing to explain to passers-by what was happening.
Potters Fields Park has been hired for a stunt to promote the launch of the Vauxhall Insignia car (sold under the Opel brand outside the UK).
The new cars will be unveiled in the park on Monday.