TOKYO >> A jigsaw puzzle featuring the Super-Kamiokande neutrino detector is gaining popularity in Japan mainly because it’s so difficult to finish.

The challenging puzzle, based on the Super-Kamiokande neutrino detector in Hida, Gifu Prefecture, has been in short supply following its release in October 2017.

The Institute for Cosmic Ray Research of the University of Tokyo released a new, even more difficult version this autumn, and it has also been enjoying good sales.

“We didn’t expect this kind of popularity. I think this is a great opportunity to make our research known to the public,” said institute director Takaaki Kajita.

Kajita used the Super-Kamiokande neutrino detector for his research into the mysterious particles that earned him the Nobel Prize in Physics in 2015.

The puzzle features an image of the Kamiokande, which has thousands of photo sensors that detect particles inside a gigantic water tank.

The 300-piece puzzle drew attention after it was first sold for $13.20 as an official product at a University of Tokyo event. All 500 sets immediately sold out.

The institute had about 10,000 additional puzzles made and sold them at seven locations around the nation, including a university cooperative and a Nagoya branch of the store Tokyu Hands.

“The puzzle is a big hit. We’ve sold more than 2,000,” said a person in charge of merchandise at Tokyu Hands. The item has sold out at some other stores.

Since the pieces all carry similar images, it is said to take three to 10 hours to complete the puzzle.