People in Japan are puzzling over who has been leaving envelopes with 10,000 yen ($82; £40) notes in men's toilets across the country.
For months, some 4 million yen ($32,750) has been found in men's rooms from the northern island of Hokkaido to the southern island of Okinawa.
Most of the money has been found in government office buildings.
Each comes with a letter saying the giver hopes the money will be "useful for your pursuit of knowledge".
The bills were individually wrapped in traditional Japanese paper with the word "remuneration" handwritten on the outside in ink, the Reuters news agency reports.
The mystery has gripped the nation, with much media speculation about the identity of the unknown benefactor.
The only thing everyone agrees on, given where the money is found, is that the person leaving them is likely to be a man.
Handwriting experts say the letters are all written by the same person, whose shaky handwriting suggests somebody elderly or seriously ill.
They also note that the writing has worsened over the months since the first set of bills was found.
"The fact that the letters end with the phrase 'please be happy' points to somebody who's unhappy themselves - who's perhaps facing up to their death and wants to give something back to the world," the Asahi Shimbun quoted one expert as saying.
The police say that money handed into them will be given to the finders if nobody claims it within six months.