I find space terrifying in general anyway, but 172 Hours really brings it to another level. There's just something about the soundless, motionless void of space that chills me to the core. Harstad harnesses the natural creepy feeling of space, the moon especially, and multiplies it by about 100 with the addition of some unknown malicious entity. I like that little hints as to what's on the moon are sprinkled throughout, but it's still unknown enough that it packs a punch with its anonymity right up until Harstad wants you to know. Although, other people may figure it out before the reveal, I think the idea isn't something overused so it's still creepy whether you know or not.
172 Hours is definitely something I'll pass along to others. I loved the thrill of the read, even if I didn't like the ending. I'll concede that it was probably the more exciting ending, but that doesn't mean I have to like it.