I will discuss arithmetic properties of K3 surfaces of type (2,2,2) in P^1xP^1xP^1, with some remarks on the Markoff-Hurwitz surfaces x^2+y^2+z^2=axyz+k (which are affine log-K3 analogues) and higher dimensional Calabi-Yau varieties given by the vanishing of a (2,2,...,2) form in P^1xP^1x...xP^1.