had been manning the Coldwater II observation post for the two and a half weeks immediately preceding the eruption. The evening before the eruption he was scheduled to be relieved by USGS geologist Don Swanson, but something came up, and Swanson asked Johnston to take his place. Johnston agreed. That Saturday, the day before the eruption took place, Johnston ascended the mountain and went on a patrol of the volcano with geologist Carolyn Driedger. Tremors shook the mountain. Driedger was supposed to camp on one of the ridges overlooking the volcano that night, but Johnston told her to head home and said that he would stay on the volcano alone. While at Coldwater II, Johnston was to observe the volcano for any further signs of an eruption. Just prior to his departure, at 7 p.m. on the evening of May 17, 13½ hours before the eruption, Glicken took the famous photograph of Johnston sitting by the observation-post trailer with a notebook on his lap, smiling. The following morning, May 18, at , the volcano erupted. Immediately, rescue workers were dispatched to the area. The official USGS pilot, Lon Stickney, who had been flying the scientists to the mountain, conducted the first rescue attempt. He flew his helicopter over the scarred remains of trees, valleys, and the Coldwater II observation post ridge, where he saw bare rock and uprooted trees. Because he saw no sign of Johnston's trailer, Stickney began to panic, becoming "emotionally distraught". Frantic and guilt-stricken, Harry Glicken convinced three separate helicopter pilots to take him up on flights over the devastated area in a rescue attempt, but the eruption had so changed the landscape that they were unable to locate any sign of the Coldwater II observation post. In 1993, while building a extension of Washington State Route 504 (also called "Spirit Lake Memorial Highway") to lead to the Johnston Ridge Observatory, construction workers discovered pieces of Johnston's trailer. His body, however, has never been recovered. Consequences and response The public was shocked by the extent of the eruption, which had lowered the elevation of the summit by , destroyed of woodland, and spread ash into other states and Canada. The lateral blast that killed Johnston started at a speed of and accelerated to . Even USGS scientists were awed. With a Volcanic Explosivity Index value of 5, the eruption was catastrophic. More than 50 people were killed