On May 29th 2006 a mud volcano, later to be named `Lusi`, started to form in East Java. It is still active and has displaced > 30,000 people. The trigger mechanism for this, the world`s largest and best known active mud volcano, is still the subject of debate. Trigger mechanisms considered here are (a) the May 27th 2006 Yogyakarta earthquake, (b) the drilling of the nearby Banjar Panji-1 gas exploration well (150 m away), and (c) a combination of the earthquake and drilling operations. We compare the distance and magnitude of the earthquake with the relationship between the distance and magnitude of historical earthquakes that have caused sediment liquefaction, or triggered the eruption of mud volcanoes or caused other hydrological responses. Based on this comparison, an earthquake trigger is not expected. The static stress changes caused by the rupture of the fault that created the Yogyakarta earthquake are a few tens of Pascals, much smaller than changes in stress caused by tides or variations in barometric pressure. At least 22 earthquakes (and possibly hundreds) likely caused stronger ground shaking at the site of Lusi in the past 30 years without causing an eruption. The period immediately preceding the eruption was seismically quieter than average and thus there is no evidence that Lusi was `primed` by previous earthquakes. We thus rule out an earthquake-only trigger. The day before the eruption started (May 28th 2006), as a result of pulling the drill bit and drill pipe out of the hole, there was a significant influx of formation fluid and gas. The monitored pressure after the influx, in the drill pipe and annulus showed variations typical of the leakage of drilling fluid into the surrounding sedimentary rock strata. Furthermore we calculate that the pressure at a depth of 1091 m (the shallowest depth without any protective steel casing) exceeded a critical level after the influx occurred. Fractures formed due to the excess pressure, allowing a fluid-gas-mud mix to flow to the surface. With detailed data from the exploration well, we can now identify the specific drilling induced phenomena that caused this man-made disaster. © 2008 Elsevier B.V. Show More