In 1917, railroad engineers at Nenana, Alaska, held a contest to guess when the Tanana River would break up. The winner won $800.

That contest has grown into an annual fundraising event. For $2.50 any Alaskan can enter; sometime in April or May a tripod will founder in the melting ice and stop a clock, and the guess that proves most accurate will win a prize. Last year the prize amounted to $279,030; in all, more than $11 million has been paid out during the contest’s 94 years.

The National Snow and Ice Data Center monitors the breakup dates as one measure of climate change in the region.