In my mostly Bermuda grass lawn in northern California I have discovered small lumps of dirt in my lawn. The lumps are about the size of a marble and are all over. Is this caused by some kind of insect? Any Ideas? The lumps are tough on bare feet when you walk across the lawn. Thanks

Thanks for the suggestion, but the mounds are a little to big for a spring rake to knock down. I've dug under some of these and found nothing, no worms, no crickets. ????
Paul