William Following up on Kirk's wonderful photo of Ezuko is a bit of geological knowledge I picked up from students. Aso has seen several eruptions over the last 50,000 years, the foremost causing a strata of impermeable granite followed later by permeable ash. This is shaped in a wedge-like formation, allowing artesian water to flow downhill until the substrate surfaces at Ezuko as well as Suizenji. (One reason why the zoo is located there is the abundance of water.) Look around there and you'll notice much granite - some of those boulders must have been hurled from a 100 kilometers away! Be glad you were not there when that happened.