In thermodyanimcs, there's a concept known as triple point: it's a combination of temperature and pressure where a substance can exist as a solid, liquid and gas, all at the same time. This is what it looks like.

It's not often people outside a lab get to see what this looks like, because you need a vacuum pump and some chemistry kit to achieve it. But it is amazing to see a fluid freeze and bubble and look like still liquid all at the same time. [YouTube]