Geothermal Mud Volcanoes are located on the east side of the Salton Sea near Niland, CA. They are located across an expanse of dried clay about the size of a football field. They range in size from a few inches to over 8 feet. The vent opening also range in size from 1/2 inch to about a foot in diameter. The volcanos bubble (128 degrees), burp, and spit mud frequently. They make a unusual sounds :). The mud oozes down the side of the volcanoes and then dries, allowing the volcano to grow even larger.
The United States Geological Survey scientists estimate them to be about 16,000 year old! I believe the property to be privately owned, however it is not fenced and multiple tourists visit the site daily.