pit
窞 centers on a deep hole or pit in the ground, often a natural or excavated cavity.
窞 combines 穴 (hole, cave) with 臽, which likely contributes the sound and reinforces the idea of a pit or trap. The exact historical development of 臽 is uncertain.
The top 穴 is a cave or hole, and the bottom 臽 looks like a person falling into a pit. Picture a deep pit opening under a cave: 窞 is a pit.
For タン, imagine a tan-colored pit in the desert: tan -> タン, and the pit is a deep hole in the ground.