空降兵

• 穴(row; gap)RADICAL
• 工(craftsman; work)PHONETIC

An empty cave with nothing inside is just hollow space.

• 阝(mound; hill)RADICAL
• 夅(descend; go down)PHONETIC

Going down from a high hill peak is the meaning of the character 降.

• 丘(mound; heap)ASSOCIATIVE
• 八(scattered; dispersed)RADICAL

A soldier holding a weapon gathers with others into a formation as solid as an earthen mound.