On March 25, 1994, about 1450 hours Hawaii standard time, a Hughes 369D, N113RS, encountered a steam cloud while hovering and collided with mountainous terrain by the Pu'uoo Vent in the Hawaii National Park, Hawaii. The pilot was conducting a local visual flight rules air taxi flight under 14 CFR 135. The helicopter, operated by Mauna Kea Helicopters, Inc., Hilo, Hawaii, was substantially damaged. The certificated commercial pilot and passenger sustained minor injuries. Visual meteorological conditions prevailed. The flight originated at Hilo, Hawaii, at 1415 hours. The pilot indicated he lost visual contact with the ground while at a 5-foot hover after being enveloped by the steam cloud. The pilot attempted to regain visual contact with the ground by descending. During the descent, the helicopter's left skid struck a rock. The helicopter rolled right and struck the ground. The passenger told park rangers that the helicopter became enveloped in a steam cloud while at a 40-foot hover. The pilot lost control and the helicopter collided with terrain. Access to the helicopter was thwarted by sulfuric fumes from the steam. The pilot and passenger walked about 2 miles before being picked up by a rescue helicopter. The pilot also indicated in his accident report that there were no mechanical failures or malfunctions with the helicopter before the accident.