Cave
Den
noun
A cave hole is a cave or den.
The reading of 洞 is one you've learned. The reading of 穴 isn't one you've learned, though. So, think about how you go inside this cave, only to find yourself covered by some red goopy liquid. You think it's blood, but luckily it just turns out to be ketsup (けつ). If you want to be less formal, you can also use the kun'yomi and say ほらあな, but then you'd never learn that reading for 穴, and where's the fun in that?
その洞穴で躓いて、足首を捻挫しました。
I stumbled into the cave and sprained my ankle.