Scooper ‘Optimus Prime’ of SuperHeroHype has taken more photos of the set at Hoover Dam and has posted the following report

I was on the Transformers shoot also and was down inside the generator room for the shots there on the first day.I did see The transformer thingy down in the generator room. Huge! It did not transform though, special effects guys working on it said it would all be done by computer effects. It had to be 25 feet tall and was on a railcar. Had some airbags under it to simulate it struggling against the straps holding it on the railcar.

We shot some scenes with Voight and S.W.A.T. teams in a tunnel going into the side of the gorge at the bottom of the dam, very spooky place with smoke effects and strange machinery lining the walls.

The photos can be seen here.