In 1998 and 1999 which was the tail end of the Aquazone theme featured the Hydronauts, a story line that involved a battle between the Hydronaut miners and the fish-like Stingrays. The set's color scheme was similar the one of the Aquanaut sets but was different in that the transparent pieces which were now exclusively transparent green instead of blue and trans-neon orange. The Hydronauts minifigures had unique masks printed onto their faces which gave them a very alien look.
Hydronauts consists of 6 sets that ran from 1998 to 1999. As of today,
6199 Hydro Crystallisation Station is the most valuable Hydronauts set with an estimated new/factory sealed value of $493.
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