Battery Gunnison is part of Fort Hancock Military Reservation. This place is quite large and there is a lot of stuff to explore there. Battery Potter is the first disappearing gun built in the U.S. Batteries Kingman & Mills are both built as regular disappearing gun batteries, and later casemated. Lots of neat tunnels to explore in there. Batteries McCook & Reynolds also have some neat tunnels. The Nike missile base there is also pretty cool. And between all the other batteries and buildings, you could easily spend all day here. Just remember to bring a mask if you go into the underground, and watch out for the fuzz.