Since 2010, Kawah Ijen volcano has been equipped with seismometers, and its extremely acid volcanic lake has been monitored using temperature and leveling sensors, providing unprecedented time resolution of multiparametric data for an acidic volcanic lake. The nature of stress and mass changes of the volcano is studied by combining seismic analyses and volcanic lake measurements that were made during the strongest unrest ever recorded by the seismic network at Kawah Ijen. The distal VT earthquake swarm that occurred in May 2011 was the precursor of volcanic unrest in October 2011 that caused an increase in shallow earthquakes. The proximal VT earthquakes opened pathways for fluids to ascend by increasing the permeability of the rock matrix. The following months were characterized by two periods of strong heat and mass discharge into the lake and by the initiation of monochromatic tremor (MT) activity when steam/gases interacted with shallow portions of the aquifer. Significant seismic velocity variations, concurrent with water level rises in which water contained a large amount of steam/gas, were associated with the crises, that caused an although the unrest did not affect the shallow hydrothermal system at a large scale. Whereas shallow VT earthquakes likely reflect a magmatic intrusion, MT and relative seismic velocity changes are clearly associated with shallow hydrothermal processes. These results will facilitate the forecast of future crises.