Whoa, Be-Gone!

Introduction: The Road Runner is chased down a roadway by Wile E. Coyote on a rocket, and the rocket's exhaust repeatedly runs into the camera during the chase, allowing the title cards to be shown. The chase begins in full force, and the Coyote gains on the Road Runner, leans off the rocket slightly, and prepares to sink his utensils into the Road Runner. Then the cartoon pauses for a red arrow and Wile E.'s Latin name (Famishius Vulgaris Ingeniusi), and then for another red arrow and the Road Runner's Latin name (Birdius High-Ballius). The chase continues down the road as Wile E. prepares to make his move, but before he can deliver the final blow, he hits his head on a tunnel arch as the Road Runner goes through it. The Road Runner and the rocket come out the other end; the bird escapes and the rocket (gets turned around) returns toward its owner. Wile E. recovers from the jar and climbs down, but soon returns upwards when he sees the rocket coming at him. He sighs with relief and dismounts the arch, but, ironically, is hit by an ACME truck. Wile E. is pacing around a mountaintop when he runs directly into the Road Runner, who beeps and causes Wile E.'s head to retract after being scared into a rock ceiling. The Coyote chases after the bird through a cloud of dust, but only manages to throw himself into thin air. The Road Runner alerts the Coyote, who promptly suffers gravity to his annoyance. He manages to climb back up the mountain again, but no sooner does he do so than the Road Runner, on another plateau, beeps again.
1. The Coyote uses a see-saw and rock to attempt to launch himself towards the Road Runner, but the rock breaks through the cliff's edge, and the Coyote slides from the top of the board down and through the hole. He falls into almost the same spot as the first time, creating a cross of coyote imprints in the ground.
2. The Coyote patches up a trampoline on the desert ground and then proceeds to man a sniper rifle. He hears the Road Runner's beep, but doesn't see him approaching. He turns around to find the bird is actually perched 20 feet behind him. So he turns the rifle around, but there's no space on the other side. Wile E. falls off the cliff again, smiling because he thinks the trampoline will save him. Not so...he busts directly through the trampoline.
3. The Coyote has now ordered a giant rubber band and ties it around two rocks, hoping to trap the Road Runner. However, it is so elastic that it pulls the two rocks together while Wile E. is still in between them.
4. Now, Wile E. lights a bunch of fireworks inside a barrel and bungee-swings the barrel out into open space above the road which the Road Runner is about to pass. But the very edge of the rock that Wile E. is standing on breaks off, causing the Coyote to swing directly below the barrel. As the rope begins to snap, Wile E. climbs up to escape the calamity, but fails to make it off in time. He falls to the ground and is smashed by the barrel. Now, the Coyote has to escape his own security measure - a nailed-shut lid. Wile E. manages to hammer off all the nails and climb out of the barrel, then hide and wait for the explosion. However, he forgot to check the lid - the fireworks are on the lid, which Wile E. is still wearing!
5. Next, Wile E. builds a high wire structure and dons a wheel-head. He struggles to get himself balanced upside-down on top of the wire (even after the Road Runner is long gone), and when he finally is able to let go of the rock, the wire (predictably) snaps, sending Wile E. crashing headfirst into the ground. Then, the wire drapes over a power line while one end falls into the Coyote's hole, resulting in the Coyote incurring a violent electrical shock.
6. The Coyote now attaches TNT to the bottom of a high bridge as he waits on the ground with the controller. The Road Runner moves towards the intended target, but stops short of the bridge. The bridge detonates and the concrete falls directly upon the poor Coyote.
7. Wile E.'s last plan is to use ACME Tornado Seeds to trap the Road Runner. Wile E. tests them by placing one next to a small cactus and shooting the requisite blast of water at it. The seed morphs into a tornado and sucks up the cacti as planned. Therefore, Wile E. drops a handful of seeds into the road just before the Road Runner turns up. Wile E. fires his pistol, but instead of firing the seeds on the road to suck up the Road Runner, it malfunctions and all the water comes out the bottom and when it lands on the jar of tornado seeds, it grows big and it makes the jar cold. The jar spin and all the words disappears and all the remaining seeds form a huge tornado and when that sucks up the Coyote very tightly, the jar is gone and takes him on the spin of his life, culminating in a journey into an army mine field. Wile E. suffers explosion after explosion as the Road Runner pulls down the "That's All Folks!" end-title card (text pre-written) like a curtain.

Question: What obstacle does Wile E. Coyote hit when chasing the Road Runner?
Answer: The top of the tunnel.