On November 18 of 1985, the tremendous eruption of the Nevado del Ruiz Volcano in Colombia, South America, led to the second greatest volcano disaster of this century, caused by lahars that surged down over towns, such as Armero , at the foot of the volcano. From January 3 of 1986, the author of this report spent one week in this area for the purpose of conducting field surveys and collecting materials. The field survey at ground level was carried out by walking up the Ruiz Volcano to an altitude of 4,700 meters. The survey from midair was performed by a small airplane, flying over the Ruiz Volcano and along the river where the lahars flowed down past such towns as Armero, Mariquita, Honda and Chinchina. Besides this midair survey of those towns, a survey by car was also carried out at the said disaster-stricken towns