First Alternative

I love Rifts, but somehow started making some distinctions. In the reading I have done, I haven't found mention of any kind of rules about Rifts underground. I recently started running a Rifts game and started having Rifts appear underground. At first, one of my players suggested that Rifts only occur above ground, but I didn't find that rule anywhere. The main reason I am running Rifts underground is because it is based in New West and Nebraska. According to the books, there is like almost no ley lines in that area. I improvised and have had, so far, only one rift that they have actually seen underground while the other remains heresay, but yeah, it occured and more than once in the same spot. So I got the big idea of dark ley lines, lines that are completely invisible because they are underground and throw no visible light like the normal ley lines. However, I only have the dark ley lines appear in places where there are no above ground ley lines. I would guess to say that these dark ley lines are kind of like anti-ley lines, but dark ley lines sounds better to me. Anyways, I was just thinking out loud and thought I should write this down since I think it a good idea. Use sparringly, but these dark ley lines are all over the place, especially in Nebraska and there is a dark ley line nexus at the ruins of Alliance, Nebraska. Happy gaming.