The Neptune Coin is located in the Weddell Sea, in Higher Ice Cave 2. This cave is located at the line between co-ordinates C-2 and C-3, at a depth of 21 meters, or roughly 70 feet. Within this coin's "blip" are also 3-4 Sea Angels. This coin is one of the twelve that the player needs to collect in order to complete the Solar System quest. In your collection book, the Coin's description states: "A gold coin engraved with the planet Neptune. Neptune is said to govern fate, secrecy, and illusion."