Want a nerdy and a little dangerous weekend? Oklahoma’s Salt Plains National Wildlife Refuge will not disappoint you. This place is home to an 11,200-acre salt plain, protected as a stopover for migratory birds.

The harsh landscape offers a host of recreational opportunities, yet the real fun is beneath the surface of the crusted salt field. The plains are the only known place in the world where you can dig for selenite crystals with hourglass-shaped, so-called, ‘inclusions.’ Geology geeks will love the experience.

But beware. The plains were not open for crystal digging from 2007 to 2009 after a Boy Scout unearthed vials of chemicals that were buried beneath the salt plains during World War II. Authorities since have determined the area to be safe for tourists, but the federal website for the refuge still carries an ominous warning. Beware of where you dig.