When dense fluid sits atop lighter fluid in a gravitational field, small perturbations grow exponentially. The growth rate σ = √(Akg/(1+kL)) where A = (ρ₂−ρ₁)/(ρ₂+ρ₁) is the Atwood number. Heavy fluid falls as spikes; light fluid rises as bubbles. This simulation uses a density field advected by an incompressible velocity field.