bubbles under the sand

I've noticed bubbles forming under my sand. Is this good or bad?

 

Good.

 

what is causing them?

 

How deep is your SB? Bacteria in the sand will convert the Nitrates into nitrogen.As more gas is created they will form bubbles which will eventually rise to the surface.Releasing too much at once is probably not a good thing but a few sand sifters like Nassarius snails or my fav. a Tiger Conch will stir small areas at a time and help release some of the built up gas. Bubbles are a good sign that your SB is working.

 

my sand bed is 3 inches. i do have nassarius snails. I do see bubbles being released sometimes. my nitrates are zero. My tank has been setup about a year. Its a 75 gallon with a 20gallon sump.

 

As Vinny implied, that's great news.