There was an area of sand near the tideline on Pobbles beach this morning with lots of these small lumps. They turned out to be air bubbles: when I pressed them I could move the air around underneath like pressing a blister. And when I punctured one, it deflated.
I've never seen anything like these sand bubbles before - and I'm an oceanographer! (That is to say, I have a piece of paper saying I have a degree in oceanography; if it weren't for that I wouldn't believe it myself. I find it hard to imagine I ever knew enough to achieve that qualification.)
Husband doesn't believe me - about the sand lumps being full of air, not about me having a degree although that is even more unlikely - but it's perfectly true.