dark spots on bottom of feet

Dr. My question is this. What would cause a dark brow spots on the bottom of feet. The last couple of weeks my boyfriend has noticed a few of them on his feet. Last night when giving a massage I notice a large brown spot about the size of a silver dollar on the pad area of his left foot. We had been walking for about six hours the day before and were wondering if it could be broken blood vessels or if it was a sign of something else. Could this be a huge liver spot? the smaller ones seem to be dark and under the skin more. This one looked as if it was like stain on his foot. Yes we did try to scrub it off first. Any answers would help.