It cuts through most of Allegheny National Forest, and is therefore sparsely populated, with the largest settlement along the road being Endeavor.

The route progresses eastward, intersecting with East Hickory Road, a dead end street that is part of old US 62.

The southeastern motion that PA 666 was following becomes a steady east, with the highway making several changes in its direction.

Hazelton Hill Road parallels to the south along the eastern-direction highway and both turn southward soon afterward.

There is a small break in the forestry, for some homes, and the highway then begins to make its loop to the north.

PA 666 now makes a steady path to the northeast along the creekbanks, and the dense forests return.

Continuing through the forestry, PA 666 begins to change to a more eastern direction, and in the process, leaving the creekbanks.

The route heads northward, running along the riverbanks once again, and enters the small municipality of Porkey, Pennsylvania.

The route continues into Howe Township and quickly leaves, turning to the northeast and into the dense forestry.

[3] This routing of PA 666 from Barnes to Nebraska, small municipalities in Forest and Warren Counties.