坑内

1. Origins, structure, and basic meaning

• Structure: 坑内(こうない) is a noun for the inside of a mine or its underground workings.
• Basic usage: Work inside a mine appears in 坑内作業, while a place of discovery appears in 坑内で道具を発見する.

2. Common pitfalls and similar expressions

• Inside a mine
  • It focuses on mine shafts and underground work areas rather than underground space in general.
• Compound and location
  • Attach it directly in 坑内作業 to classify the work, and use 坑内で for the place where an action occurs.