We will discuss a correspondence relating principally polarized complex abelian surfaces to certain K3 surfaces of high Picard rank -- the K3 surfaces polarized by the lattice H+E8+E7. Although the correspondence has a Hodge-theoretic origin, it can be explained in purely geometric terms by means of a special isogeny of K3 surfaces. This framework also allows one to obtain explicit formulas for the correspondence in terms of genus-two Siegel modular forms. This is joint work with Adrian Clingher.