Nine years ago, a rice paddy in eastern Java suddenly cleaved open and began spewing steaming mud. Before long, it covered an area twice the size of Central Park; roads, factories and homes disappeared under a tide of reeking muck. Twenty lives were lost and nearly 40,000 people displaced, with damages topping $2.7 billion.

The disaster, known as the Lusi mudflow — a combination of lumpur, the Indonesian word for mud, and Sidoarjo, the area where the eruption took place — continues today. A “mud volcano,” Lusi expels water and clay rather than molten rock. Such eruptions occur around the world, but Lusi is the biggest and most damaging known.

Scientists have debated the cause for years, and two intensely argued hypotheses have emerged: Some believe an earthquake set off the disaster, others that the mudflow was caused by a company drilling for natural gas.

Researchers largely relied on computer models and comparisons with other earthquakes and mud volcano eruptions. But recently scientists in Australia, the United States and Britain uncovered a previously overlooked set of gas readings collected at the drilling site by Lapindo Brantas, a natural gas and oil company, in the days before the mudflow began.