The vacuum properties of lattice QCD with staggered quarks are investigated by an efficient simulation method. I present data for the quark condensate with flavor N f = 0, 1, 2, 3, 4 and many quark masses, including the vacuum energy in the chiral limit. Obvious sea quark effects are observed in some parameter space. I also describe a mechanism to understand this and a formula relating the chiral condensate and zero modes.