A 2.2‐kton nuclear explosive was detonated underground at one end of a 122‐m long, 1‐m i.d. horizontal tunnel at the Nevada Test Site. The purpose of the experiment was to study the hydrodynamic flow of energy down a tunnel from a nuclear explosive. This report describes the experimentation utilized to record luminosity of the shock front for the first 15 &mgr;sec and time of arrival of the shock front at specific positions along the tunnel. The observed temperature, determined from shock‐luminosity data and the blackbody radiation assumption, dropped from 11.8 to 2.3 eV at times of 3.2 and 18 &mgr;sec respectively after the peak energy output of the nuclear explosion. The shock wave traversed the 122 m in approximately 4 msec, with a velocity ranging from about 13 cm/&mgr;sec near the shot point to approximately 1 cm/&mgr;sec at the far end.