If I had to guess I'd say that the big craters that you are seeing are subsidence craters from underground nuclear testing. Here's the Wikipedia link: http://en.wikipedia.org/wiki/Subsidence_craters

By the way, I'm a NDP alum (2002) and I stumbled on your site (Wikipedia for NDP led me to irishweek.net which led me here) and I have to say that I'm really impressed. Good luck with the site and school, don't get hurt playing with your coilgun (back in my day the closest we got were potato guns... kids these days...).