  We establish a structure result for the universal abelian variety over the
moduli space A_5, in terms of discriminant curves of conic bundles over a del
Pezzo surface. In particular, this gives a very simple unirational
parametrization of the boundary divisor of the moduli space A_6 of principally
polarized abelian varieties of dimension 6. Using this we derive a lower bound
on the slope of all effective divisors on A_6.
