Here's another one: http://trendbeheer.com/2006/09/16/interdimensional-bling-portal-in-dronten/

Both in the Netherlands (Purmerend, Dronten) and yes it an artefact on the ground: a roof reflecting the light of the sun.

Explanation (in dutch, with pictures) - someone went to check the exact location: http://www.hippopotame.nl/index.asp?id=800