This is really intriguing. Looks like an underwater steam vent.
Yes I am intrigued by it too. This is my second attempt at freezing flowers. The first time I just used Rose petals and buds. The bubbles were small and normal looking. I am wondering if the lavender I used this time emitted a small amount of gas.... or maybe it was the oil. I will try again another time with something different. 🤔😏