Pure Hell! A network of military roads, double track, rocky very technical singletrack, smooth singletrack through some forest. Very tough climbs, both single and doubletrack. Don't bike alone. There are many trails out there, I'm not sure how many miles. I couldn't find a good map, but there may be one. Big climbs. Not for beginners. Even if you take a short loop, the climbs will test you. There are even sounds of artillary fire, and you might even see some helicopters, or other military training. I'm not sure if this place is open year round. Call Mud, Sweat, and Gears in Lawton (580-510-9224) or LETRA (580-442-5541).
Take I-44 north of Lawton, OK to Exit 45. Take HWY 49 west for about 0.5 mi, and turn left (south) on Apache Gate Road on Fort Sill Military Base. Be prepared to be searched, and have ID. Directly after the gate, turn right (west) and follow the signs to Lake Elmer Thomas Recreation Area (LETRA). The trial head isn't marked, but it by the tent camping, and starts off by having to climb the hill on the west side of the campling.
http://www.singletracks.com/item.php?c=1&i=1155