There is some seriously weird shit going on. When I breached the first cavern, I also discovered an empty magma pipe. "Weird, what's going on here?" I thought to myself. So I went to explore. Or rather, a kitten went to explore. I threw it in the emptied pipe to see what all the fuss was about and it fell. And continued falling. And falling. It fell 109 z-levels before hitting a solid floor. But a couple of stories before it fell to it's doom, it discovered the magma sea. I was pleased that it had served a purpose, and thought no more of it. Except, this was a rather odd magma sea. It wasn't quite full. It was flowing. Almost as if it was draining into something. Impossible! I said to myself. And while the kitten kept on falling, it discovered the lowest feature. But how? I wondered. Upon closer examination, it was clear that the magma sea was draining in the HFS naturally. I had just breached it, and the magma was pouring in the circus. Acting quickly, I locked up all entrances to my fort and continued.
A couple of minutes ago, I was bored and greedy and decided to go exploring further: I hadn't found caverns 2 and 3 yet. So I dug down, avoiding opening up C1 and quickly discovered C2. I dropped in a dog and closed the entrance. I went deeper and deeper and came across C3. And half of it is submerged in magma. It's ten z-levels above the magma sea my kitten discovered, but not completely 7/7. This means that C3 is connected to the sea, which is connected to the circus.