Boulder is a typical Egyptian obstacle, but it occurs in other worlds. The boulder can fall from the above (or from the bottom, it's one of ??? anomaly).
The special type of trap activates a running boulder chasing Hero. Avoiding this piece of rock is rewarded with Catacomb raider achievement.
Boulders make a danger in Happy Hill, Egypt, ??? and Forest. Hero has to watch out for boulder traps in Egypt and Forest.