For some time I have been wondering about this. It's a picture from my (paved) garden. And from time to time these kind of sand formations appear between the tiles. And I wonder what kind of force is creating them. When I am flatten them with my shoe till it's even again as it should be then after a few days it comes back again. I went through all kind of scenarios, I think I finally know, although unproven yet. What's your guess?
My guess is some kind of insect tunneling left-overs. Instead of compressing with your foot, try scooping it up and see what's under.
Insects were my first guess also. Obviously something is moving beneath and those formations are spread all over in my garden. But since I never saw any insect crawling out nor was there any increase of insects I started to doubt the insect theory. A local mole seems to be too farfetched. I considered the theory of a possible minor gas leak gas finding its way to the surface. I tried with fire, no result. Then I noticed something, it's on the picture right above the formation, the little clump of grass. Grass (God's carpet for the Earth) is almost indestructible, it grows everywhere also in the sand between the tiles and from time to time I remove them pulling them out including the roots and the remaining little hole is filled with sand again. So aside the insect theory I have now a second theory. Not always do I manage to pull out the grass with root completely and a remain may stay. It's not unlikely the grass is recovering itself, is growing and pushing the sand upstairs. And now I will have to be patient, no longer automatically flat the formation and wait if a blade of grass will appear. So much excitement