Through experimental investigation, a thin subwavelength cavity resonator was physically realized using a bilayer structure composed of air and a negative permeability metamaterial structure one unit cell in thickness. We designed and built the metamaterial slab with periodic metallic ring structures and measured the spatial electric field magnitude in a cavity formed from this slab and a region of air, showing that a subwavelength cavity can be realized. The measured electric field magnitude distribution in the cavity matched very well with effective medium theory, showing that even a slab one unit cell in thickness may be effectively equivalent to a thin homogeneous medium as far as the construction of a sub-wavelength cavity is concerned, provided that the unit cell size is significantly smaller than the free space wavelength.