We demonstrate terahertz microcavity lasers at lambda=112 mum with ultra-low current thresholds of 4 mA and with mode volumes of less than one-cubic-wavelength. Confinement in the longitudinal direction is obtained using almost-circular micro-disk resonators. The guiding properties of surface-plasmons are exploited to guide the mode with the metal contact. The devices laser up to 70 K in pulsed mode, and up to 60 K in continuous-wave.