North Dakota is located in the center of the North American continent, in the plains east of the Rocky Mountains and at the border to Canada. The borders to the north and south are longitudinal circles, and thus appear straight on many maps. The western broder is a longitudinal circle and also straight. Only the eastern border depends on a geographic feature, the Missouri River.
North Dakota is a rural state without big cities. Fargo, at around 70,000 inhabitants, is the largest city. Absolutely flat in the east, rolling prairie with scattered lakes and sloughs, getting rougher to the west with steep buttes and rugged canyons. This are badlands with their extraordinary wildlife, and most of the caves of the state. In the Killdeer Mountains are layers of carbonate rocks at the surface, but no much karstification and no karst caves. All caves in North Dakota are a result of erosion or slope failure.
The two biggest caves of North Dakota are Medicine Hole and O'Brien Ice Caves. Medicine Hole on the Medicine Hole Plateau in the Killdeer Mountains, Dunn County, is a tectonic cave formed by a block which broke away from the mountain. The surrounding rock is the Arikaree Formation (Oligocene-Miocene) which consits of tuffaceous carbonates and sandstones. O'Brien Ice Caves, also known as Wonderful Ice Caves or just Ice Caves, are located on the northern edge of Billings County. Another tectonic cave, which was formed by the detachment of blocks from the 10m thick fluvial sandstone cap rocks. The blocks have come to rest at various angles against each other, which has created some small chambers. The ice cave effect is based on the opening to the north, the ice trap effect and the good insulation by the sandstone.
There was some mining in North Dakota, especially for coal, which resulted in numerous abandoned mines. Today some of them start to collaps producing huge areas with enormous dolines. Those areas, as the mines, are very dangerous and should be avoided. There are several open cast lignite mines, which are open to the public. With an estimated 351 billion tons of lignite, North Dakota has the single largest deposit of lignite known in the world. And there is an estimated 25 billion tons of economically mineable coal.