When you get to the volcano area (which should be in about the first four hours), the treasure chests start giving Ice Crystals. Ice Crystals "Freeze a volcano."
Now that should have been enough to clue me in. As you're exploring the volcano, most of the ground you walk on is red hot lava. And sure enough, walking on red hot lava is bad for your health (or HP, to be more exact!)
It seems to vary a bit but you'll probably lose around 16-20 HP per step - unless you use an Ice Crystal to freeze the lava. One Ice Crystal freezes one screen, until you go to the next screen and then it resets. (And I made it about halfway through the volcano before I discovered this - no wonder every time I entered battle all my characters had 1 HP.)
