Top: hole, cavity 穴 (It is covered 宀 and a narrowing path 八 leads into the cave.), bottom: nine 九 (Here as left part of “pigeon 鸠 はと”, where on the left the bird ナ is shown, whose head 乚 is on the ground when pecking.
Mnemonic
At the hole is the head on the ground when investigating.
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