Mount Hekla, an active volcano, is situated about 100km east of Iceland's capital, Reykjavik. It is 1,491 m high and is said to be the most active volcano among the many volcanoes in Iceland because recently it has been erupting approximately once every 10 years. 　When Hekla erupted in 1300 AD, it killed 600 people. After that, Icelanders feared it and referred to it as "The Gateway to Hell" in the Middle Ages.　 Much later, Mount Eyjafjallajökull, which is situated to the south of Mount Hekla, had a major eruption in April 2010 and emitted large amounts of volcanic ash, which lead to air travel disruptions in Europe.
　On September 2nd, 1845, Mount Hekla had a major eruption. At the time, the Danish government, which was ruling over Iceland, commissioned a scientific study on Mount Hekla that included Professor Bunsen from the University of Marburg, Germany.
　Bunsen is definitely not an unknown name. The Bunsen burner, which is a common piece of equipment in science labs, was named after him.
　Robert Wilhelm Bunsen (1811-1899) was an accomplished scholar with various achievements in the field of chemistry (such as the discovery of cesium and rubidium). He was also greatly interested in the field of geology when he was younger. The foreign authorities probably heard about this and, therefore, invited him to participate in the study. In 1846, Bunsen participated in the expedition, which took place a year after Mount Hekla erupted. He conducted field studies, sampling and analyzing volcanic gas and volcanic rocks. He also discovered a geyser.
　At the time, water from geysers was believed to come from volcanoes and were thought to be different from rainwater. However, Bunsen obtained water that is similar to geysers by boiling rainwater and placing volcanic rocks he collected in it. From this, he concluded that the geyser water in Mount Hekla originated from rainwater. This was the first ever modern research related to the origins of hot spring water. Later, Bunsen's theory was also called the theory of circulating water. The research on the origins of hot spring water continues today.
　Bunsen was also intrigued by the mechanism of geyser eruption. He measured the vertical water temperature changes in geysers, conducted studies in both theory and model experiments, and proposed the widely known critical column theory. That was in 1847. After that, geyser-related studies were continued by other researchers. In Japan, Kotaro Honda and Torahiko Terada conducted studies using the geyser in Atami as their study model, 60 years after Bunsen.
　The aforementioned research conducted by Bunsen in Iceland is said to be the beginning of modern hot spring science. Bunsen taught at the University of Heidelberg starting in 1852. He died in Heidelberg at the age of 88. Yôan Udagawa, mentioned previously, passed away in 1846 at the age of 48, when Bunsen was conducting his field studies.

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