A hollow has formed in this tree.
A small crab is hiding in a cavity in the rock.
The squirrel hid the acorn in the hollow.
穴 is a general word for any hole or opening, while 虚（うろ）specifically refers to a hollow cavity inside something like a tree.
洞（ほら）usually means a cave or larger hollow space one can enter, while 虚（うろ）is typically a smaller hollow, such as inside a tree trunk.
Glossogobius olivaceus; a species of goby
Standard kanji spelling for 'hollow' or 'cavity'.
Kana spelling, sometimes used when the kanji is not needed.
Rare kanji variant for 'hollow' or 'cavity'.
Alternate kanji spelling, also used for 'hollow'.
The origin is uncertain, but it may be related to the idea of emptiness or hollowness. The kanji 虚 and 空 both carry meanings of 'void' or 'empty', while 洞 means 'cave' or 'hollow'.