The Sani Pass is a stretch of extremely undeveloped road that links South Africa's KwaZulu-Natal with the mountain Kingdom of Lesotho. It's neither for inexperienced drivers nor anything but a 4x4.
Although the Sani Pass is only about 5.6 miles long, it climbs a whopping 4,370 feet in that distance – when you reach the top, you're at over 9,400 feet in elevation. The road is essentially a dirt path of switchback after switchback, so it's imperative that you have not only a 4x4 but an experienced driver behind the wheel.
The Sani Pass leads to Lesotho—the country's border is at the summit—and because of the dangerous conditions it can sometimes be closed due to inclement weather. Improvements are slated to be made to the road, paving parts of it, but the planning for these improvements has been going on for years already.
Traveling from South Africa into Lesotho means ascending the Sani Pass, so that trip is best done with an experienced local guide. Vehicles that aren't capable of making the ascent are turned back by border agents.