While on holiday in the Karoo, South Africa, a friend took me to see this interesting formation. We think they look like nests, that range in sizes from 50cm by 35cm to 120cm by 80cm. I have attached a couple of photos. I hope somebody can help me. Thanks for having a look.
I agree with Brettarcher on this being a geological formation. And, from the shape of it, my best guess would be that you've a lump of sedimentary rock where a nodule used to 'nest' inside it, hence the smooth, rounded cavity.