  We show the existence of infinitely many admissible weak solutions for the
incompressible porous media equations for all Muskat-type initial data with
$C^{3,\alpha}$-regularity of the interface in the unstable regime and for all
non-horizontal data with $C^{3,\alpha}$-regularity in the stable regime. Our
approach involves constructing admissible subsolutions with piecewise constant
densities. This allows us to give a rather short proof where it suffices to
calculate the velocity and acceleration at time zero - thus emphasizing the
instantaneous nature of non-uniqueness due to discontinuities in the initial
data.
