We prove that the 3D stable Muskat problem is globally well-posed in the critical Sobolev space H˙2∩W˙1,∞ provided that the semi-norm ∥f0∥H˙2
Nearby in the stack
is small enough. Consequently, this allows the Lipschitz semi-norm to be arbitrarily large. The proof is based on a new formulation of the 3D Muskat problem that allows to capture the hidden oscillatory nature of the problem. The latter formulation allows to prove the
H˙2
a priori estimates. In the literature, all the known global existence results for the 3D Muskat problem are for small slopes (less than 1). This is the first arbitrary large slope theorem for the 3D stable Muskat problem.