This lecture series on cutting-edge researcher frontiers covers newly emerging framework for unsaturated soil mechanics. It describes the physics laws and principles beyond the classic soil mechanics but necessary for understanding fundamental soil properties and mechanical behavior under variably saturated conditions. Specifically, the lecturer addresses the following basic questions on soil’s behavior: Why soil water density is not equal to unity? Why pore water pressure under unsaturated could be compressive up to a few GPa? Why the convention definition of matric suction being the pore pressure difference between air and water fails to define soil matric potential? Why the popular use of the pore pressure difference between air and water as a stress variable is unphysical? Why the classic definition of effective stress by Bishop is incomplete in describing effective stress under unsaturated conditions? The lecturer presents a unified theory of soil sorptive potential to answer these fundamental questions that have been failed by the classic soil mechanics and the prevailing unsaturated soil mechanics theories. An effective stress equation, unifying Terzaghi’s, Bishop’s, and the lecturer’s earlier equations, is presented for soil under all saturations.