Muddy road - Motorcycle (Laos)
This is the "road" the connects Phonsavan to Paksan, via Thathom. There is 80 Km like that (and generally much worse), with up to 70cm (2 feet) deep mud or water. This road is NOT passable in the rainy season, due to rivers with no bridges. It took me two full days, and that was the most difficult road I ever drove through. At several points the muddy water was so deep that the wheels and engine were entirely submerged.
The only trucks that can go on this road are large military 4x4 or 6x6 trucks like this one.