In 1943 a new volcano arose in Hokkaido. The Japanese government managed to keep it a secret for several years.
New mountains don’t appear that often. It is a rare and precious event for volcanologists when they do, because it gives them an ideal opportunity to study the geological processes underpinning their work. So a series of earthquakes in northern Japan beginning on December 28, 1943, should have been a happy scientific occurrence. The ground under a field of wheat began bulging upwards, and in June 1944 it erupted. Lava pushed upwards, forming a new lava dome in the fields: Shōwa-shinzan. It kept growing for more than a year, sometimes lifting by as much as two metres a day.
Alas, at the time Japan was at war. The government for some reason thought that a volcanic eruption in the middle of Hokkaido could be interpreted as a bad omen, or undermine confidence in the war effort. So Japan’s newest volcano was kept a secret from everyone but the local inhabitants. No chance to Do Science on it, no chance to observe its progress or evaluate its dangers. Fortunately for us, one of those local inhabitants decided to take it into their own hands.
Masao Mimatsu was the area postmaster. He strung up a series of fishing lines across his view of the emerging volcano; those lines served as a reference point for its growth and development. He sketched the silhouette of Shōwa-shinzan over the course of several months in the so-called Mimatsu Diagram. In doing so he gave us the only accurate record of the birth of this new mountain.
Today Shōwa-shinzan is just under four hundred metres high and is part of a national park. You can see it in the third link below.