Hollow, Cavity, Cave
noun
A sky cave is a cave with sky (open space) in it. This is a hollow, but can also refer to a cavity.
くうどう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
白熊が木の空洞の中でうたた寝をしていると、近くに木こりがやって来ました。
A polar bear was snoozing in the hollow of a tree and a lumberjack soon came along.