究

Top cave radical — a covered hollow in the earth, the kind of dugout dwelling early Chinese lived in. Indexes 究 in the cave family with 空 empty, 窗 window, 窝 nest. Sets the scene for the original meaning: poking into a deep hole to see what is inside.

Bottom 九 supplies the sound with a tone shift: jiǔ → jiū. 九 also faintly hints at completion or thoroughness — nine being the highest single digit in classical numerology, a number suggesting reaching the bottom of something. Together with the cave: "going to the end of the cave."