Geologists say the area along Highway 87 that collapsed in March, causing millions of dollars in repair, may be the site of an ancient landslide.
"The preliminary results show that it could be an ancient landslide area as old as 10,000 years, at the earliest to one million, at the oldest," said Arizona Department of Transportation public information officer Bill Williams.
"So under the pine trees, there could be a zone that has been moving, which is just fascinating to hear from geologists."
ADOT has already spent $3.6 million to repair damage that occurred March 21 when a landslide dumped dirt onto the roadway and buckled the pavement 20 miles south of Payson.
Crews installed a 4-foot wide drainage pipe, concrete pilings and monitoring devices to measure any future movement. Williams said movement on the hill is negligible.
Phase two of the project, creating a retaining wall with the pilings is expected to start soon and continue into the winter.
"Right now we are going back to the contractors and looking at the budget and seeing what we want to do," Williams said.
"We may be looking at spending $18 million, so how can we wisely spend the taxpayers' money."
Knowing the history of the area may change how engineers fix the hill and prevent future landslides.
"The geologists are helping us look at the bigger picture," he said.
In early August, geologists from Oregon walked all over the mountain and will be coming back to give their final assessment.
"We have a pretty good idea of how to keep Mother Nature away from the highway," Williams said.
"What we want to check off is if they have any other ideas."