Mima Mounds are low, domelike, symmetrical natural mounds - mainly composed of loose, unstratified, often gravelly sediment. They have a diameter of 3 to 50 m, and a height of 0.2 to 2 m.
They are found mainly in the northwestern United States - though similar structures have also been found on every continent except Antarctica. They sometimes occur in groups of thousands.
Over 150 years or so of research, there are now several theories to explain their origins - none of which has been generally accepted. (As their construction tends to vary, it's currently thought that different mounds may have differing origins.)
The main theories are that they were formed by ;
See : BBC Earth
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