--- The dielectric in an aluminum electrolytic capacitor is a microscopically thin layer of aluminum oxide which is maintained on one of its aluminum plates when the polarity of the voltage across the two plates, and the electrolyte between them is applied in the correct direction. If the voltage is appled in the wrong direction, the electrolyte will deplete the oxygen in the coating, reducing the oxide coating to elemental aluminum and the two plates will short-circuit through the electrolyte, destroying the capacitor.
-- John Fields