Start at the second checkpoint, located on the second large tower. Drop down onto the ramp below, then try to slowly inch off of the next ramp so that you land in the gap behind the ramp, falling onto a hidden platform. Drive off of this platform and you'll see that the wall blocking the path will fall. Make sure it doesn't hit you!
From here, drive across the ground until you reach the edge of the cliff. Drop off of the side and you'll land on another platform. Go forward a bit and you'll be ambushed by squirrels hiding in the roof.
Nobody is quite sure how the squirrels got into the machinery or why they are so irate, but they will launch an attack at anybody driving a motorbike through the lower level.
What we can confirm is that they are squirrels. Given that they are located in numerous tracks, they may not be as significant as other easter eggs.