Summon avatar in brutals?
 

I don't know exactly how it happens, but it tends to happen quite often in brutals. I will be using summon avatar, and a BigWorm will pull me underground. When I attack it with avatar (or someone else does) I freeze, and then just have to sit there until they cave is destroyed. Here is a pic:

I usually attack the worms with avatar when I'm far away - get swallowed once, memorize the location, then send Bile in. As you always look downwards in a worm, I never bothered getting an avatar above me...