Large Headron Collider
For any science nerd, CERN is like Mecca.
They just completed the 27km LHC and are about to start colliding things together. I wish I could give a better explanation than that, but I really don't understand much of anything about nuclear physics. Hopefully it won't create a giant black hole and destroy the universe...
The Boston Globe has a great collection of giant/amazing photos of the LHC in Geneva, Switzerland (how I miss thee!).