The LUSI mud volcano in the sub-district of Porong, Sidoarjo, East Java, Indonesia started to erupt on 29 May 2006. An almost continuous eruption of a mixture of mud, water and gas has occurred around this area since this date. The eruption triggered vertical and horizontal ground deformation. From June 2006 to December 2010, 14 global positioning system campaigns were conducted to observe the ground deformation using c. 50 stations sparsely located up to 10 km from the eruption centre. Field observations of cracks, terrestrial laser scanning and geo-electrical measurements have also been used to infer the ground deformation signature around the LUSI mud volcano. More than 150 pairs of interferograms generated from 66 ALOS PALSAR images from June 2006 to December 2009 have also been used to study the ground deformation caused by the LUSI mud volcano. The LUSI mud eruption began only 200 m from where the Lapindo Inc. oil company was drilling for oil and gas. The drilling may have pierced a deeper high-pressure zone, causing an underground blow-out of the drillhole into a hydrofracture. Alternatively, the magnitude 6.3 Yogyakarta earthquake, which was located c. 275 km from the eruption site and occurred two days before the LUSI eruption, may have shaken the area sufficiently to cause the eruption by reactivating a fault in the region and liquefying the mud. These two hypotheses for triggering the mud volcano have been argued vehemently and still remain controversial. The ground deformation signatures provide important clues to understanding the trigger for the eruption and to solve this controversy. Co-seismic fault reactivation has its own typical ground deformation signature. This study used global positioning system and InSAR techniques, as well as field observations of cracks, terrestrial laser scanning and geo-electrical measurements, to determine the signature of ground deformation around the LUSI mud volcano and to explain the triggering mechanism.