National Wildlife Refuges in Oklahoma
Designated by Teddy Roosevelt as the Wichita Mountains Game Sanctuary in 1901, the Wichita Mountains National Wildlife Refuge was one of the very first National Wildlife Refuges established in the United States. It's in the heart of the Wichita Mountains, an eroded mountainous landform suspected to be some 500 million years old. That same landform runs westward across the Texas panhandle to the Guadalupe Mountains region where it extends several thousand feet above ground.
I drove in one day and did the unthinkable (for someone who was then living in the foothills of Colorado's Rocky Mountains): I went mountain climbing in a wilderness area in Oklahoma. There are three wilderness units in the Wichita Mountains and one of them (Elk Mountain) offers a reasonably easy walk-up on a well-established trail with a huge amount of excellent bouldering available at the top.
Ozark Plateau NWR is closed to the public to protect the cave-dwelling species that live there.
Salt Plains NWR is sometimes closed to the public as it used to be a military bombing site, and once upon a time someone digging there for gypsum crystals came across a buried kit previously used for the testing of nerve gas agents (that's how our government used to dispose of such things: just bury them in the dirt in (what was) the middle of nowhere). Every now and then some unexploded ordnance will show up, too.
I asked about that when I paid a visit to the property in the fall of 2012 and the wildlife officer on duty told me that had all been cleaned up. With all the gas and oil drilling (and fracking) happening in the Salt Plains area, the wetlands have almost dried up. There is some stream flow still in Sand Creek but the natural artesian flow from the northeast side of the property that usually provided most of the surface water to Salt Plains was used almost exclusively for the gas and oil business for almost two decades.