This trail is often called one of the most beautiful in Oahu. Maunawili translates as "twisted mountain", and the trail is aptly named. Although a challenging ride up from sea level, once you reach about 1000 ft, the singletrack is cut into the side of the Ko'olau range. The trail winds in and out of a number of steep valleys to stay at about the same elevation, and small waterfalls abound in these valleys.
The trail passes through a tropical forest high above the Pacific. The singletrack is extremely narrow and technical in places and there are a zillion, usually wet, roots along the way. Expect to get muddy even on a "dry" day. (Do not ride this trail if it is raining!) The dense forest makes you feel like you're a million miles from nowhere, but occasional breaks in the forest cover offer breathtaking views of the very steep and very green eastern face of the Ko'olau Mountains with the big blue Pacific below.
This is a challenging ride with some cliffs. Even expert riders will probably have some hike-a-bike sections. Be prepared for a long slog.
You can make this into a big loop with the the Old Pali Highway or the new Pali Highway (HWY 61) with Kalaninaole Highway (HWY 72), and Kumuhau St. Alternatively, there is a short connector that connects to the suburb of Manunawili which allows you to avoid the highways.