Caesium
Caesium is extremely reactive in air and water. Caesium rapidly oxidizes in air and can spontaneously combust (randomly catch on fire) at any moment. For this reason, it must be stored in kerosene or a mineral oil, like other group one elements (Lithium, Natrium, Rubidium, and Francium.) In water, Caesium violently reacts to make Caesium Hydroxide (2CsOH).  The Caesium sinks for about one second, then explodes. The explosion is over 50 times the size of the element dropped in the water, and the explosion is enough to break a common Pyrex Beaker, Flask, or Test Tube.  You can find a video of the reaction here.