"That's the big question," said Scott De Leon, county public works director. "We have a dormant volcano, and I'm certain a lot of things that happen here (in Lake County) are a result of that, but we don't know about this."
Other development on similar soil in the county is stable, county officials said.
While some of the subdivision movement is occurring on shallow fill, De Leon said a geologist has warned that the ground could be compromised down to bedrock 25 feet below and that cracks recently appeared in roads well beyond the fill.
"Considering this is a low rainfall year and the fact it's letting go now after all of these years, and the magnitude that it's letting go, well it's pretty monumental," De Leon said.
County officials have inspected the original plans for the project and say it was developed by a reputable engineering firm then signed off on by the public works director at the time.
"I can only presume that they were checked prior to approval," De Leon said.
The sinkage has prompted county crews to redirect the subdivision's sewage 300 feet through an overland pipe as manholes in the 10-acre development collapsed.
Consultant Tom Ruppenthal found two small leaks in the county sewage system that he said weren't big enough to account for the amount of water that is flowing along infrastructure pipes and underground fissures, but they could be contributing to another source.