The actual Oklahoma high point at 1516m. Sharing a name with the Half-Life facility makes it a Wikipedia pilgrimage for gamers.
From Wikipedia
Black Mesa State Park is an Oklahoma state park in Cimarron County, near the western border of the Oklahoma panhandle and New Mexico. The park is located about 15 miles (24 km) away from its namesake, Black Mesa, the highest point in Oklahoma. The mesa was named for the layer of black lava rock that coats it.