cave

1. Origins, structure, and basic meaning

• Structure: 洞 has 9 strokes and appears in words for hollow spaces and caves.
• Basic usage: 空洞 is read くうどう, and 洞窟 is read どうくつ.

2. Common pitfalls and similar expressions

• A hollow space and a cave
  • 木の幹に大きな空洞ができていた means that a large hollow had formed inside a tree trunk.
  • 暗い洞窟へ入った means entering a dark cave. 空洞 is not always a cave that people enter.
• Location
  • 道路の下に空洞が見つかり means a cavity was found beneath the road, a different location from a hole on its surface.