6199 Hydro Crystallization Station is a LEGO Aquazone set released under the Hydronauts subtheme in 1998. It contains the large, underwater base of the Hydronauts, a small submarine, several sea creatures and five minifigures, including three Hydronauts and two Stingrays. This is the only base of the Hydronauts as well as the largest and most expensive set of this Aquazone subtheme.