You never quite know when a given volcano is going to erupt — but you can bet on it. Ireland’s biggest bookmaker, Paddy Power, jetted to fame among geologists in early January, when it announced its latest novelty bet: which of a handful of famous volcanoes around the world would be the next to powerfully erupt.

The idea began with an eruption. In mid-December 2009, Mount Mayon volcano in the Philippines began spewing ash clouds and lava. The eruption attracted the attention of some eager betters, who lobbied for the opportunity to take their chances on other twitchy volcanoes. Within a week, Paddy Power obliged, determining eruption odds for about a dozen volcanoes around the world.

Since then, the list has continued to grow, as betters request odds for their favorite volcanoes; the odds for each volcano on the list, according to Paddy Power spokesperson Ken Robertson, are determined with the help of an anonymous “London-based geologist.” The payoff will come from the first volcano to erupt with a Volcanic Explosivity Index (VEI) of at least 3, on a scale ranging from 0 (non-explosive, like many Hawaiian eruptions) to 8 (mega-colossal, with long-lasting effects on climate and landscape, like the eruption of Indonesia’s Toba 74,000 years ago). VEI is based on how much ash and lava the volcano erupts, the height of the ash cloud, and other factors.

Japan’s Mount Unzen is still the odds-on favorite at 3-1, with other famous volcanoes (the Congo’s Nyiragongo, 4-1, and Hawaii’s Mauna Loa, 9-1) close behind. But a dark horse candidate emerged in March, when an Icelandic volcano near Eyjafjallajökull glacier, 75 meters east of Reykjavik, erupted with a VEI of 1 after remaining quiet for 200 years. The volcano was quickly added to the list the next day, with Paddy Power offering odds of 28-1. “Quite a few people started getting into it,” Robertson says.

In mid-April, when Eyjafjallajökull erupted again, hundreds of betters rushed overnight to place bets on the volcano, prompting Paddy Power to slash the odds to 5-1 to reduce the company’s potential liability. In late April, as the eruption continued, Paddy Power offered a second betting option: When the eruption would stop.

Although most of the betting at Paddy Power is on sports events, politics and celebrity gossip, many of the site’s novelty bets comprise a grim lineup of science-related speculations, such as “Extinction Specials” (a recent bet was on the polar bear count by the World Wildlife Fund as of Dec. 31, 2011). Another novelty bet involves the setting of the Doomsday Clock, a giant clock at the University of Chicago that since 1947 has allegorically told the time of man’s proximity to global destruction (currently set at six minutes to “midnight”). Betters can gamble on when it will next be reset, and whether the reset will put us closer to doom (five minutes, 11-8 odds) or farther away (seven minutes, 4-1 odds).

For anyone in the U.S. who is mulling over whether to place a volcano bet, however — don’t bother: Paddy Power is not currently accepting bets from people across the pond, where it is still illegal in many states to engage in online gambling.