Since 2010, the Kawah Ijen volcano has been equipped with seismometers and its extremely acid volcanic lake has been monitored using temperature and levelling sensors, providing unprecedented time resolution of multi-parametric 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 observed during the strongest unrest ever recorded by the seismic network at Kawah Ijen. The distal VT earthquake swarm that occurred in May 2011 was likely the precursor of a volcanic unrest, in October 2011, thereby causing the increase in shallow earthquakes. VT earthquakes opened pathways for the fluids to ascend by increasing the permeability of the rock matrix. The following months were characterized by two periods of important 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 an important amount of steam/gas, were associated to the crises although the unrest did not affect the shallow hydrothermal system at a large scale. Whereas shallow VT are likely reflecting a magmatic intrusion, MT and relative seismic velocity changes are clearly associated to shallow hydrothermal processes. These results will facilitate the forecast of future crises.