Many people in Japan swear by the medical benefits bathing in onsen, or hot spring baths. As one family found out recently however, a visit to an onsen in the winter can have fatal consequences.

According to police hydrogen sulfide gas fume being emitted from the ground near a hot spring claimed the lives of four family members in Yuzawa, located in Akita Prefecture.

The police were alerted to the tragedy when they received an emergency call reporting that a group of people had collapsed near a parking lot. Responding, they found Yasushi Matsui along with his wife and two young daughters, all of whom were unconscious. The four were rushed to a hospital but the mother and daughters soon died. The father is still alive, but remains unconscious and in serious condition.

Investigators said an outlet for hydrogen sulfide gas was located near a parking lot in the area. It had been covered with snow, but gas coming out had melted the snow, leaving a hole in the ground about 1.5 meters deep. One of the children reportedly tried to recover a flying disk that had fallen into the hole, and Rie Matsui and the other child had apparently fallen in one after the other while trying to help.

Officials say that sulfurous fumes emitted by hot springs are normally blown away by winds during the summer. In the winter however, escaping gas can collect in holes that form naturally under snow and accumulate to dangerous levels.