If it's an underground map, a skybox is pointless. Putting a skybox around it only increases compile time. Just make sure you seal the map properly. A func_detail is a brush entity, so select a brush, hit ctrl+T (or select the "to entity" button on the right) and it'll default to a func_detail. Brush entities can be changed to stuff like func_breakable, which indicate breakable stuff, and much more.