Video: In Laugarvatn, Iceland, the ground itself is an oven. Throughout the town, which lies within the country's Golden Circle, hot springs bubble up through the sand at up to 100C. While tourists soak away their cares in those same springs, old-fashioned bakers bury pots of simple rye dough in the earth to cook. Twenty four hours later, the rúgbrauð comes out moist and spongey, almost cake-like.
It's far from the only dirt or ash bread made across the world (the Australian damper comes to mind) but it certainly seems like the cleanest.