Snow remains near the peak of the 1,613-meter-high mountain. The swamp, about 50 meters in diameter, is said to look like a huge "dragon eye" when seen from above.
When the snow over the center of the swamp melts in a doughnut shape, the dragon eye "opens." This year the dragon eye opened on June 7.
Many tourists came from neighboring prefectures to see the naturally occurring phenomenon during the balmy weather on June 9. From the parking lot near the peak, they climbed a cobbled hiking route to reach the swamp.
"I am happy to have seen the dragon eye this year," said Tadao Fujine, 68, a part-time worker from Hanamaki, Iwate Prefecture.
"I would like to post the photos on my Facebook account," he added.