Upon triggering, a huge cylinder that is covered in spikes, comes rolling toward the group from a large hidden hatch. There is room above the cylinder to jump over, but not to either side. Characters get to make Dex roll or Saving Throw to avoid, otherwise anyone in the way takes 3d6 of damage. The cylinder then passes into another large hidden hatch on the other end, and lots of clunks and other mechanical sounds imply that the trap has reset itself. Optional: The GM could put tiny alcoves into the walls on either side, to aid in survival, and allow for bonuses to evade the trap.