in early 1980. On March 15, a cluster of tiny earthquakes rocked the area around the mountain. For six days, more than 100 earthquakes clustered around Mount St. Helens, an indication that magma was moving. There was initially some doubt as to whether the earthquakes were precursors to an eruption. By March 20, a magnitude 4.2 earthquake shook the wilderness around the volcano. The next day, seismologists installed three seismic recorder stations. By March 24, volcanologists at the USGS—including Johnston—became more confident that the seismic activity was a sign of an impending eruption. After March 25, seismic activity drastically increased. By March 26, more than seven earthquakes over magnitude 4.0 had been recorded, and the next day, hazard warnings were publicly issued. On March 27, a phreatic eruption took place, ejecting a plume of ash nearly into the air. Similar activity continued at the volcano over the following weeks, excavating the crater, forming an adjacent caldera, and erupting small amounts of steam, ash, and tephra. With each new eruption, the plumes of steam and ash from the volcano rose, eventually climbing to . By late March, the volcano was erupting up to 100 times per day. Spectators congregated in the vicinity of the mountain, hoping for a chance to see its eruptions. They were joined by reporters in helicopters, as well as mountain climbers. On April 17, a bulge (or "cryptodome") was discovered on the mountain's north flank, suggesting that Mount St. Helens could produce a lateral blast. Rising magma under Mount St. Helens had veered off to the north flank, creating a growing bulge on the surface. Final signs and primary blast Given the increasing seismic and volcanic activity, Johnston and the other volcanologists working for the USGS in its Vancouver branch prepared to observe any impending eruption. Geologist Don Swanson and others placed reflectors on and around the growing domes, and established the Coldwater I and II observation posts to use laser ranging to measure how the distances to these reflectors changed over time as the domes deformed. Coldwater II, where Johnston died, was located just north of the mountain. To the astonishment of the USGS geologists, the bulge was growing at a rate of 5 to 8 feet (1.5 to 2.4 meters) per day. Tiltmeters installed on the volcano's north side displayed a northwest trending tilt for that side of the mountain, and a southwest trending tilt