How do museums secure lawn-sculptures?

Does anyone know?

For example: In San Francisco, the De Young museum has various outdoor sculptures, including a number of ceramic apples. Though they're not meant to be sat on, they're just the right size, so people do. So far, they haven't toppled over or moved, so they're pretty solidly fastened.

So how are they secured to the ground? There's a steel plate under each one, but what's holding that? Concrete?

I need a character to pull one out. He's got superhuman strength, so he can do it, but I want to know what he'll have in his hands when he does.

I suppose another option would be to insert some kind of saw just under the sculpture and cut it loose from the plate. But I don't want any damage to the sculpture...

Thanks, all! I think I'll try talking to them.