Sakurajima — an active volcanic island near Kyūshū — Magma Soda. It's your basic sugary soda — quite good and not too sweet — but spiced with Japanese chilies. It's somewhat spicy, though not so much that it dominates. Flavour as much as heat if you will.
No, the interesting thing is that the chili heat combined with the carbon dioxide bubbles really irritate your throat and nose. Not so it hurts or anything, but the moment I took the first sip I burst into a coughing fit. Another sip, another cough. The third sip hit the back of my nose and triggered an impressive sneeze attack. My wife tried a little and started coughing as well.
I've never before had so much fun just drinking soda. But, I do understand why they're selling it only in small 200ml bottles, and why they advise you to keep it away from children. Try it, if you happen to come across it.
#1 Not that I actually drink much soda, mind you. I loved it when young, but it's usually too sweet for me now.