Where do the air bubbles come from?

I have just returned from holiday and while I was standing with the sea lapping round my knees I noticed that my feet sank into the sand a little deeper the longer I stood in the same place but each time I pulled my foot up to the top of the sand (but still under the water ) air bubbles rose to the surface. I had not introduced the air by lifting my foot out of the water so where did it come from as there seemed to be a limitless supply of air bubbles being released from the sand, which had been under water for several hours.