An ultralarge ring He–Ne ring laser gyroscope, UG-2, with area 834 m2 and dimensions 39.7×21 m2, has been built underground at Cashmere Cavern, Christchurch, New Zealand (latitude -43.575°). Earth rotation is sufficient to unlock it, giving a Sagnac frequency of 2.18 kHz. Supermirrors are used with transmission ∼0.18 parts per million (ppm) and optical loss unexpectedly high at ∼200 ppm per reflection. The cavity Q is 1.5×1012. Residual Sagnac frequency error caused by backscatter coupling is measured as ≪2 parts in 108. Its best stability is achieved for an averaging time of ∼2000 s, for which the Allan Deviation of the Sagnac frequency is 0.08 mHz, or four parts in 108 of Earth rotation rate. The dominant processes generating the residual rotational noise are, for times ≪10 s, microseismic ground movements, and for times ≫1000 s, mechanical movement of the mirror assemblies, which act to change the geometrical dimensions and tilt. At all averaging times the residual rotational noise is well above the limit imposed by quantum phase fluctuations. It is concluded from comparisons among many large ring lasers that the excess mirror losses arise from high order aberrations, and UG-2 may be larger than the optimum size.