In the first room, at the faraway left top corner, is a small giveaway button. Shoot it, and a door will open.

When you use the crane to smash a door, there is a discrete button in the control room. Push it, and the floor to your left opens.

Behind the bridge, that can dip into the lava, is a bit of raised floor. Jump down from it, into the hole in the floor, and take the lift up to get the heavy machinegun. Don't end up in the lava. You won't like it.