User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-5043481-20130226183601/@comment-4212769-20130226184403

Well, a first thought: Maybe those rocks are not as dense and solid as they seem to be? They can carry your weight, but are easy to break through. (similar to sand)

For examply Shogun: In the caves of the Swamps the moisture/water may made the ground porous while the heat in the Volcano may have the same effect by vaporizing the water in the ground, leaving some hollow ground that make the ground more porous.

But well, it's just a first thought.