(Joint with Samuel Grushevsky, Gabriele Mondello, Riccardo
Salvati Manni) We determine the maximal dimension of a compact
subvariety of the moduli space of principally polarized abelian
varieties Ag for any value of g. For g<16 the dimension is g−1,
while for g≥16, it is determined by the larged dimensional compact
Shimura subvariety, which we determine. Our methods rely on
deforming the boundary using special varieties, and functional
transcendence theory.