Since it roared to life in May 2006, a mud volcano near Indonesia's coastal city of Sidoarjo has swallowed homes, rice paddies, factories, and roads, killing 15 people, displacing 40,000, and harming the livelihoods of many more. As the ongoing eruption nears its 5th anniversary, observers wonder whether it will ever stop. The answer: Not anytime soon. A new study predicts the volcano will continue spewing significant amounts of mud for another 2 decades. A second study forecasts that it could grind on as long as 87 years.

The mud volcano has inflicted a punishing blow to the region of Java island 700 kilometers east of the capital, Jakarta. Nicknamed Lusi, a contraction of lumpur (Indonesian for mud) and Sidoarjo, the volcano has so far disgorged 144 million cubic meters of mud, some of which now covers an area roughly twice the size of New York City's Central Park. Much of the mud has been diverted to a nearby river, where it has formed a new 83 hectare island and extended a natural delta. Compensation and mitigation have cost at least $767 million, according to Humanitus, a nongovernmental organization in Melbourne, Australia, that is studying the disaster's social impact. That is a fraction of the real economic toll, which is still being tallied.

Lusi may be a harbinger of disasters to come. "Like a volcanic eruption, a mud eruption is just the effect of geological activity, and I'm sure in the future another mud volcano must erupt in this region," says Soffian Hadi Djojopranoto, a geologist with the Sidoarjo Mudflow Mitigation Agency. "We need very serious research to understand this phenomenon."

Despite being the most intensely studied mud volcano ever, scientists have failed to agree on the cause of the eruption, which began in the early-morning hours of 29 May 2006. Mud suddenly started gushing out of vents 200 meters from a rig drilling an exploratory gas well. Drilling logs indicate problems with the well several hours before the eruption, and many scientists believe there was an underground blowout. Others, however, suggest that a magnitude-6.3 earthquake that occurred 2 days earlier and 280 kilometers away activated a local fault. Despite the uncertainty, the Indonesian government pressured the Bakrie family, majority owners of the drilling company and one of the country's wealthiest families, to foot most of the bill for compensation and mitigation.

Debate now centers on how Lusi's plumbing works. "The most important piece of work now is to estimate the longevity," says Richard Davies, a geologist at Durham University in the United Kingdom. That will determine if mud-handling countermeasures are sufficient. Dueling hypotheses have led to different forecasts. Davies argues that the eruption is driven by pressurized water from a deep aquifer in permeable material beneath an impermeable rock layer. He argues that the wellbore pierced the impermeable rock, allowing water to gush up and sweep overlying mud to the surface. Modeling this scenario using combinations of known quantities, such as total ejected mud volume after 1 year and 3 years and assumed parameters, including aquifer size, Davies and colleagues arrived at an estimated longevity of 26 years, published online on 24 February in the Journal of the Geological Society. They also predict that the ground around Lusi will subside up to 475 meters from its original elevation, with mud filling the crater.

Others augur that Lusi will be kicking around far longer. Michael Manga, a geologist at the University of California, Berkeley, contends that pressure and fluid originate not in the deep aquifer but in a shallower mud layer. In a paper in review, his team predicts that an ever-widening circle of subterranean mud will get sucked into the volcanic system and pushed to the surface. The model "is a new way of thinking about how eruptions work," Manga says. His team estimates a 50% chance that the eruption will last 40 years and a 33% chance that it will drag on for 87 years.

The predictions are getting a mixed reception. Peter Flemings, a geologist at the University of Texas, Austin, has not seen Manga's results, but he says his "gut feeling" is that tapping into a large permeable aquifer, as Davies proposes, would produce the volume of material spewing from Lusi. The "absolutely critical assumption," Flemings says, is the aquifer's size—and calculating that from limited data, he says, "is fraught with uncertainty." Davies's subsidence projections, meanwhile, "look big," says Heri Andreas, a geophysicist at the Institute of Technology Bandung in Indonesia. GPS surveys of ground deformation show that after an initial period during which the ground was sinking up to 4 centimeters per day, subsidence has tapered off to just several centimeters per year.

For more robust projections, says Manga, "we need more and better data." And more is at stake than scientific models. Long-term social, ecological, and infrastructure programs can't be planned "until this geological phenomenon is better understood," says Humanitus Director Jeffrey Richards. Humanitus is organizing a May symposium in Surabaya at which Richards hopes experts will forge a consensus on what studies are most likely to reveal Lusi's geological secrets. Davies would like to see a well drilled into the aquifer some distance from Lusi to measure pressures. Other options are 3D seismic surveys of the subsurface.

Numerous efforts to plug the volcano have failed. Fortunately, the mud flow is now manageable, says Djojopranoto. After peaking at 180,000 cubic meters per day in early 2007, the rate has tapered to 10,000 cubic meters per day. A system of 6- to 7-meter-high earthen dikes encloses some 700 hectares of ponds where mud and water is collected and then pumped into the Porong River, where it is adding to a natural delta downstream. The impact on the Porong has been minimal, given that it historically carried heavy sediment loads from magmatic volcanoes upstream, Djojopranoto says.

Environmentalists claim that authorities are understating some of Lusi's ill effects. Studies by nongovernmental organizations in 2007 indicated that high sedimentation was smothering marine life, particularly bottom-dwelling creatures like snails, says Pius Ginting of the Indonesian Forum for Environment. An ongoing concern, he says, is the mud's toxicity, which he claims is laden with carcinogenic polyaromatic hydrocarbons—a contention that Djojopranoto says has never been independently verified.

In Lusi's vicinity, the mitigation bureau has rerouted roads and resettled most families. Mud volcano tourism is providing income, says Djojopranoto, but "not enough to revive the economy." Even after the eruption ends, Lusi may erupt periodically or ooze mud for centuries. "On east Java, we have mud volcanoes that have been active for hundreds of years," Djojopranoto says. None, however, compare in size, in societal harm, or in the puzzles that Lusi continues to present to scientists.