Some say they are formed by the wind. Some claim they are Native American burial mounds. Others believe ancient earthquakes might be responsible, or even extraterrestrial activity.
But it now seems that the answer to a question that has divided scientists for years - what exactly causes 'Mima mounds'? - is a lot closer to home.
They are, according to new research, the work of generations of gophers, tiny burrowing mammals native to the United States.
The mounds cover huge tracts of land, and are found across the western US from California to Washington state, as well as occurring in other countries. They vary in size from 2-3m wide to 50m, and can be 2m tall. Patches of mounds can contain millions and span several miles.
Lead researcher Dr Manny Gabet, of San Jose State University, told BBC News: "The big mystery surrounding Mima mounds is that, until now, nobody really knew how they formed.
"Over the past couple of hundred years, people thought they might be Native American burial mounds, or they were caused by earthquakes or glaciers. Some people even suggested extraterrestrials."
His work, however, shows that gophers are responsible, piling up earth as they burrow into the ground. Dr Gabet and his colleagues analysed the way in which the rodents burrow, and showed that they created the mounds in waterlogged areas in order to stay dry.
The size of the mounds had led experts to dismiss the idea that gophers were responsible, but Gabet's research found that the animals built the mounds over incredibly long periods of time, with each generation adding more soil.
To arrive at the answer, Dr Gabet built a computer simulation of dozens of 'digital gophers' burrowing in the same area, and saw patterns emerging that immediately resembled the Mima mounds. The animals are unique in that they push earth up and out behind them.
According to the computer model, a large mound takes between 500 and 700 years to form. Speaking to LiveScience, Dr Gabet explained that "What's really cool about this is scaled by body size, these are the largest structures built by any mammal not including humans. In terms of effort, it would be like a single person building the pyramids."
Each mound's size corresponds roughly to the range of a single gopher. When a mound reaches a certain maximum - about 50m wide - it stops growing, indicating that the gophers know when it is high enough to escape any surface water.
Mima mounds are common in areas that experience heavy winter rainfalls that create 'vernal pools' - seasonal bodies of water that then soak the surrounding soil. Gophers live underground, so need to create habitats that remain dry all year round.
The mounds are named for Mima fields in Washington, where a large number of the mounds can be found. Carbon dating analysis has revealed the structures to be several thousand years old.