Marvel Comics continuity
Bombshell used a cerebro-shell to force hydroelectric engineer Ricky Vasquez to bring Megatron into the energy plant at Hoover Dam, as part of the Decepticons' plan to steal the plant's energy and transport it to Cybertron via the space bridge. In response, Optimus Prime sent the newly built Aerialbots to hinder the Decepticons. Ricky Vasquez eventually regained control of his body and fired Megatron at the space bridge, saving the plant.
Transformers 2007 film
Hoover Dam has always been a dam intended to provide power. It was not the secret base of Sector Seven, because Sector Seven does not exist. At no point in time did it hold any robotic lifeforms or alien technology. There is nothing to see! These aren't the droids you're looking for. You may go about your business. Move along.
Hoover Dam plays a vital role in this game, even though it is visited only once. Six missions take place in this region of the game.
In In Through the Back Door, Jazz must locate a radio tower without being noticed by S7. This means no transforming, no shooting, and no hitting guard troops! Then, after climbing the tower, he must avoid S7 once more while locating the Military Hangar. While there, he must destroy S7 drones.
In The Heist, Bumblebee must destroy three Decepticons without shooting (you can't shoot), then find and collect his weapon chip, then retrieve the Allspark, then escape from the dam.
In Bombing Run, Ironhide must throw six bombs into the water while fending off two Decepticon drones. Above all, he must accomplish this in less than three minutes.
In Interference, Optimus Prime must defeat Brawl, who I suspect took the place of Bonecrusher. He hates that. In order to defeat Brawl, just run around him while shooting, then pick up and smash tanks on top of him. Raar.
Again, six missions take prominence in Hoover Dam. Most of them involve Megatron in some way or another.
In Communication Breakdown, Brawl must destroy three satellite dishes, then place four bombs along the Dam. However, before he can place the fourth, an Autobot takes it. Brawl must kill the Autobot. Finally, he must bring a tank back to the specified spot.
In Tag Team, Blackout must find Barricade and carry him over the dammed lake, then destroy a tower, then drop Barricade in. Now Barricade must find out where Megatron is.
In Megatron's Return, Create-A-Bot must protect Megatron from being destroyed by S7 and Autobots while being thawed out. Oy.
In Get the Chip, Blackout must retrieve Megatron's weapon chip from a bunch of convoys without letting any of them escape. Wow, tough.
In Ambush, Megatron must destroy turrets, then kill off Jazz. Easy enough, hey?
In an alternate timeline, Megatron uses the former Sector Seven base as a prison for Optimus Prime - who he has frozen, for he is petty. A tunnel link to the dam can be accessed in Denver.