The Sidoarjo mud flow (commonly known as Lumpur Lapindo, wherein lumpur is the Indonesian word for mud) is the result of an erupting mud volcano in the subdistrict of Porong, Sidoarjo in East Java, Indonesia that has been in eruption since May 2006. It is the biggest mud volcano in the world; responsibility for the disaster was assigned to the blowout of a natural gas well drilled by PT Lapindo Brantas, although company officials contend it was caused by a very distant earthquake that occurred in a different province.

At its peak it spewed up to 180,000 cubic meters (240,000 cu yd) of mud per day. By mid August 2011, mud was being discharged at a rate of 10,000 cubic meters (13,000 cu yd) per day, with 15 bubbles around its gushing point. This was a significant decline from the previous year, when mud was being discharged at a rate of 100,000 cubic meters (130,000 cu yd) per day with 320 bubbles around its gushing point. It is expected that the flow will continue for the next 25 to 30 years. Although the Sidoarjo mud flow has been contained by levees since November 2008, resultant floodings regularly disrupt local highways and villages, and further breakouts of mud are still possible.