The ground state of electronic systems changes from metallic to insulating with increasing disorder. Disorder localizes the electronic wavefunctions and also limits the electrons ability to screen their mutual Coulomb interactions. By reducing the density of carriers in such systems, both the effects of disorder and Coulomb interactions increases, thereby, driving the system from a metallic to insulating behavior. Until recently, a metal-insulator transition (MIT) was believed to exist only in 3D. Surprisingly, recent transport experiments discovered the same MIT phenomenology in 2D systems. However, despite of our extensive knowledge of their macroscopic characteristics, the microscopic properties of the MIT in 2D remains unknown.

   In my talk I will describe our recent local compressibility measurements of the MIT in 2D. Our measurements reveal a striking microscopic evolution from a continuous liquid phase to a discrete insulating phase. In contrast to the continuous compressible phase, the new discrete phase consists of microscopic charge configurations that are compressible only at discrete values of the local density.  The discrete phase appears already on the metallic side of the MIT and its volume increases on account of the continuous phase when the density is lowered. Furthermore, we find that the individual charge configurations, that comprise the discrete phase, interact via quantum mechanical tunneling and via mutual Coulomb interaction.