… something big and rocky with a Space Force base in it

CHEYENNEMOUNTAIN

This Colorado landmark houses a military installation and is known for its rugged, mountainous terrain.

This clue hints at a well-known military installation inside a large, rocky mountain. "Cheyenne Mountain" is home to a Space Force base, and the answer combines the words into one without spaces. The reference to "something big and rocky" points directly to the mountain itself, while "Space Force base" confirms its modern military significance.