The managers of a Rotherham nature park say they are "appalled" after part of a sculpture was stolen from the site.
One of the crossbeams was taken from the Steel Henge structure at the Centenary Riverside park on Wednesday.
The sculpture comprises a circle of metal plinths and beams which were excavated when the park was created.
The Wildlife Trust for Sheffield and Rotherham, which manages the park, is concerned that the thieves might return to take more of the sculpture.
Rob Miller of the Wildlife Trust said the theft had been reported to police.
"We're amazed and appalled at the theft of part of the Steel Henge sculpture from this innovative and popular nature park.
"We're asking people who see anything suspicious or unusual happening on the site to please inform the police immediately.
"It would be a huge pity to lose any more of the sculptures that reflect the area's distinctive and proud heritage."
Centenary Riverside Nature Park is a joint venture between Rotherham Metropolitan Borough Council and the Wildlife Trust for Sheffield and Rotherham.
It forms part of the £2bn Rotherham Renaissance programme to regenerate the town centre and bring the riverfront back into use.