In recent years, the method of vortex layer cascades has successfully been applied to construct finite-time singularities in multiple fluid equations (3D Euler, Boussinesq, IPM, g-SQG...). In this talk, I will motivate and describe the method and sketch how it can be applied to the 2D Boussinesq and 2D non-homogeneous Euler equations. In both systems, the growth of the vorticity is driven by an accumulated hysteresis effect triggered by the cyclic deformation of the density layers, whose dynamics is governed by an infinite chain of degenerate pendula. Our work also settles the previously open question about global existence of classical solutions of the (forced) 2D non-homogeneous Euler equations.