SAN DIEGO - Some say they were created by earthquakes. Others say aliens made them.
Now, a researcher says he has discovered the secret behind one of the West Coast's biggest geological mysteries.
From Puget Sound to San Diego, the sight of them is strange and unexplained: dirt mounds known as Mima mounds, rising as high as 8 feet tall and 30 feet wide.
For thousands of years, they covered the mesas of San Diego County, including most of Kearny Mesa and Otay Mesa.
Theories for the mounds range from earthquakes to floods to aliens.
"I would definitely say it's an enduring mystery," said local geologist Pat Abbott.
It is a mystery that Emmanuel Gabet, a researcher from San Jose State University, believes he has solved.
Abbott is not that researcher but he brought a 10News crew to some Mima mounds in the Miramar area.
Almost all of the local mounds and nearby vernal pools have disappeared because of development.
"Here were see some of the burrows," said Abbott as he pointed some out.
Could the burrows be the home of the true architect of the mystery mounds -- the pocket gopher?
The gopher theory is based on the work of former San Diego State University professor George Cox, who studied gophers living under local mounds and used colored pellets to track dirt movement.
He found that gophers moved dirt uphill when the ground underneath got wet.
"A safe and well-drained site is absolutely essential for their nests," said Cox. "Over decades and centuries, that gradually builds the mound."
Using Cox's research and computer modeling, Gabet showed the creation of the mounds over a thousand-year period by gophers.
"But that doesn't tell you, did they build the mound to begin with, or did they simply occupy the mound and then modify it?" said Abbott.
Other scientists have questioned whether a simulation is definitive proof.
Cox says it is the best explanation for a sight that has for so long been heaped in mystery.
Some scientists, including Abbott, still maintain there are multiple causes for Mima mounds.
Other theories include wind erosion and glaciers.