This talk focuses on the regularization by nonlinear noise for a class of partial differential equations that may only have local solutions. In particular, we obtain the global existence, uniqueness and Feller property for stochastic 3D Navier-Stokes equations, which provide positive answers to some longstanding open problems in this field. Moreover, we discover a new phenomenon that for a potentially explosive deterministic system, an appropriate intervention of nonlinear noise can not only prevent blow-up but also lead to the finite time extinction of the associated stochastic system. Our main results have various applications, including stochastic p-Laplace equations with heat sources, stochastic surface growth models and stochastic quasi-geostrophic equations.