Given the task definition and input, reply with output. In this task, you are given a paragraph, and your task is to construct a question from the given passage. Construct question in such a way that (i) it is well explained in the paragraph, (ii) the words may/may not be present in the paragraph, (iii) it should start with what/how/describe. Avoid creating a question that (i) is just relevant to one or two sentences, and (ii) not relevant to passage. 

Some of the rock in the ground is made of limestone. Over long periods of time rain can hit the limestone. And cause a acidic reaction. The limestone will slowly disintegrate. A large gap with be where the limestone was. Result in the cave. 

How do caverns form?