DD2 has a pretty deep one, freaked me out, I think I posted about it when she was NB. She has a reeeeally high bum crack too! lol
We were never referred to an ultrasound or anything though, she is quite healthy. Just have to make sure I wipe her bum really well and keep it clean and dry.
Apparently it can be a symptom or indicator of spina bifida a few posts said, or can be just totally normal.

I hope all is well, I know none of that is probably any help, but thought I might post so you don't stress - it can just be a dimple.