Probably rusted. Those grease trap lids are usually built like manhole covers.
“The grease trap is a six-foot deep, in-ground container.
Sadie was playing in the area, which has picnic tables and a grassy area where children play, an official said.
It wasn’t the first time the Andrews family had visited the ice cream shop for the children to play and have treats.
Lee County Coroner Bill Harris said Sadie stepped onto the lid of the grease trap, which apparently wasn’t locked or secured, and fell in when the door opened.
The door closed on top of her, he said.”
http://www.al.com/news/index.ssf/2017/10/sadie_andrews_auburn_alabama_b.html