The Siegel Modular Variety of Degree Two and Level Four, by Ronnie Lee and Steven H. Weintraub Let $\mathbf M_n$ denote the quotient of the degree two Siegel space by the principal congruence subgroup of level $n$ of $Sp_4(\mathbb Z)$. $\mathbf M_n$ is the moduli space of principally polarized abelian surfaces with a level $n$ structure and has a compactification $\mathbf M^*_n$ first constructed by Igusa. $\mathbf M^*_n$ is an almost non-singular...