洞穴

1. Origins, structure, and basic meaning

• Structure: 洞穴 is read as ほらあな and is a noun for a natural hollow or cave in a cliff, mountain, or the ground.
• Basic usage: Finding a cave appears in 洞穴を見つける, while entering it appears in 洞穴に入る.

2. Common pitfalls and similar expressions

• A hollow that can be entered
  • It suggests a natural interior space that a person or animal can enter, rather than a small opening on a surface.
• Difference from 洞窟
  • The meanings are very close, though the comparison term is more commonly used for a larger cave formation.