[He lets loose another laugh, dipping the end of his paddle down into the water, leaning over with the ever-vigilant eye out for passing manatees. No luck so far. Sigh.]

'Innermost wall?' Didja live somewhere with boundaries like that? [His head tips to the side a little--and he keeps from tipping into the water. Oops, that was close.]

Oh, that. Well. Tokyo being a huge city and all, people often throw things into water supplies and they get all polluted, n'stuff. Not to mention some companies are real cheap about waste disposal, and tossing things into waterways is 'cheaper'. It's illegal, but hard to get in trouble for something if no-one knows you're doin' it.

[A short sigh.] But hey, there's always springs and rivers in the countryside like this t'look forward to.