About a week ago we got a tip from a local California resident by the moniker of “FASTBACK67,” who was out and about around the Encino Hills area – “NW of intersecting freeways 101 and 405″, according to our Screen Rant “Bruce 2.0 GPS system” – where he stumbled across something VERY interesting and, being a smart cookie, connected the dots to Iron Man 2, which is currently halfway into its shooting schedule.

And just what did Mr. FASTBACK67 discover? A huge outdoor set complete with a massive green screen, built into the famous Sepulveda basin – but what is it for?

Here’s how FASTBACK 67 stumbled upon the alleged Sepulveda set of Iron Man 2:

“I live near this rather large dam… It’s abandoned now but they still film stuff there.  A couple of weeks back they were doing some construction for a movie called Rasputin(?) so I didn’t pay it any heed, then that same night I read somebody’s blog and they said they were on the Iron Man 2 set in a different city and they referred the set as the picture Rasputin so I went back the next day to take some pics and the guards chased me off. But at the time they were just stacking storage bins and building the set, now it looks like there getting ready for some filming. The pictures in the back of the dam are about 8 to 10 stories high so you can get a bit of perspective.”

“Rasputin” is indeed the super-secret code word for Iron Man 2 when the film is being shot in public locations. Probably to avoid incidents just like this: pesky bloggers and such. Well, sorry IM2 – nowhere left to hide in the age of cellphone cameras!

Speaking of pictures, FASTBACK 67 was a savvy snoop and clicked us a couple pics of the set being built in the Sepulveda basin. The first one you’ve already seen above, here’s a second, closer look at it:

click image for larger size

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UPDATE: Thanks to the Marvel Movies Facebook Group, we have an aerial view of the Sepulveda set of Iron Man 2, which is now rumored to be for a scene involving Tony Stark (Robert Downey Jr.) throwing a rock concert. Click each image for a larger version :

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And here’s a video of the set, courtesy of Stuff We Like:

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Alright, ladies and gents, let the speculation begin: What kind of scene or sequence would they shoot at Sepulveda basin? Does a rock concert scene sound about right – a literal stage for a final showdown between Iron Man and Mickey Rourke’s Whiplash? Or perhaps the speculation is wrong and the set is for something different altogether? And what’s the deal with the massive green screen? Obviously they want the basin to look like something else, but what? Mr. Favreau, sir, can you spare a hint for a nosy blog site? We would very much appreciate it…

Also, if you haven’t seen it, check out the first promo poster for Iron Man 2:

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The floor is officially open for further speculation.

Iron Man 2 hits theaters on May 7, 2010.

Source: FASTBACK67

Poster Source: Collider

Update Source: Marvel Movies Facebook Group

Update Image Source: Flicker