海溝

1. Origins, structure, and basic meaning

• Structure: 海溝 is read as かいこう and means “ocean trench”.
• Basic usage: It denotes a deep ocean trench marked on a map and the trench bottom toward which a probe descends.

2. Common pitfalls and similar expressions

• 深い海溝 / 海溝の底
  • Use 深い海溝 for the whole feature and 海溝の底 for its deepest part; it is an ocean-floor feature, not an ordinary valley.