[RoosterRedux]

Yep. I think it was the USS Nimitz that first picked up a tic-tac entering our atmosphere at 80,000 ft (which is the upper limit of the shipboard radar IIRC) and tracked it down to sea level in 7/8 of a second (60,000+ mph). By the way, it came to an abrupt stop at that speed...and such a stop would have obliterated any known man-created object.

Side note: I am not saying our upper atmosphere starts at 80,000 ft, just that that's the altitude at which ship board radar was said to pick up the object.

[Seruzawa]

This sort of activity has been observed since the 40s. It used to be very common around bases with nuclear weapons.

[JohnnyP]

Ok, IF the aliens can stop on a dime with no apparent harm to themselves, why are they killed when they crash into the side of a cliff?

And why would they crash, if they are transmedium? They should glide through the granite face of El Capitain like butter.