Physics of cylindrical base flows from subsonic to supersonic speeds at zero angle of attack are computationally investigated with large-eddy simulation and Reynolds-averaged Navier-Stokes hybrid methodology. Time-series and time-averaged characteristics of base flows show three peculiar characteristics at subsonic, transonic and supersonic speeds. Normalized time-averaged base pressure decreases in proportion to free-stream dynamic pressure at subsonic speeds of Mach numbers less than 0.8, begins to fall rapidly at transonic speeds and gradually reducing its value toward an asymptotic value at supersonic speeds of Mach numbers beyond 1.5. Normalized RMS base pressure fluctuations gradually decrease with increasing free-stream Mach number at subsonic and supersonic speeds, but the fluctuations rapidly increase at transonic speeds. Characteristics of base flowfield are dominantly influenced by the flow phenomena of unsteady vortices shedding from free shear layers at subsonic speeds, front and rear local shock waves oscillating along free shear layers at transonic speeds and oscillations of free shear layers and the recompression shock wave at supersonic speeds. High pressure region behind the base is one of the key factors in determining the base pressure and its strong Mach number dependency is significantly affected by the dominant flow phenomena at each speed range. Strouhal numbers of the major peaks of base pressure fluctuation energy correspond to the Strouhal number of the dominant flow phenomena at each speed range. Mechanisms for the substantial base pressure fluctuation energy are the pulsing or flapping of the reverse flow to the base.