Caltrops

Caltrops are small, sharp metal objects prepared as a hazardous obstacle to slow progress and cause injury to the unwary adventurer. Frequently encountered in dungeons, they consist of four metal spikes arranged in a way that, when spread across the ground, lay with one point upright and pierce the vulnerable feet of any who may step upon them. They appear in Ultima VII and Ultima VII Part Two.

Description
Caltrops are not strictly classified as a trap—thus the spell Destroy Trap cannot remove them—and are easily disposed of by hand once discovered. However, there is a second variety of caltrop that is made invisible by magic, becoming more difficult to detect in the poorly lit environment of a dungeon.

Trivia

 * Caltrops may be considered a successor to bear traps, which served in a similar capacity, albeit more difficult to remove.