Question

insert dashes where needed.
the explorers needed torches to light their way through the dark ice cave its walls were more than twenty feet thick and blocked out the light of the sun.

Explanation:

A pair of em dashes (formed by two hyphens each) is used to set off an additional, parenthetical clause that provides extra detail about the ice cave. We place them after "cave" to start the clause and before "its" is not correct—wait, correction: we place one dash after "cave" to introduce the descriptive clause about the cave's walls, and a second dash before the final part to close the parenthetical? No, actually, the clause "its walls were more than twenty feet thick and blocked out the light of the sun" is a non-restrictive, explanatory phrase that elaborates on the dark ice cave. So we use em dashes to separate this clause from the main sentence.

Answer:

The explorers needed torches to light their way through the dark ice cave -- its walls were more than twenty feet thick and blocked out the light of the sun.