What are these bubbles in my sand?

[dejaknu]

The tank is about 4 months old. I just noticed these bubbles forming under the sand. Any ideas? I let one go up to the surface and it is indeed some gas.

 

[mtgibson2]

It is gas that is released by bacteria living in the sand bed. Bacteria eat and release gas as a byproduct.

 

[dejaknu]

Thanks! I was hoping for a more detailed answer though, if anyone has one. I know the nitrogen cycle and other things are happening in the tank but it I've never seen tanks with actual bubble pockets. So just wondering if anyone has an idea of what process specically generally does this and if it's a problem to be corrected.