A sunken grade is a place where the road literally sunk down, either into some sort of sink hole or because the underlying ground has shifted, notes MarieGale.com.
Sunken grades occur when the earth moves slowly under the road and creates a depression in the roadway.
The term "grade" is used in several different ways with respect to roads and highways. The "sub-grade" lies beneath a roadway and is used as a supporting base. "Grading" a road means to smooth out the roadbed with earth moving equipment during the construction phase.
The grade of a highway is a measure of its incline or slope. The amount of grade indicates how much the highway is inclined from the horizontal. See more at Encyclopedia.com - Grades, Highway.