The Hydra Coin (one of the one-hundred Constellation Coins in Endless Ocean: Blue World) is found in Ciceros Strait. However, this coin can be tricky to obtain, as it is inside of a hidden tunnel only accessible at night, because it is blocked by an undine in the day. The tunnel is located at coordinates F-6.
- The Hydra (the beast which the constellation on this coin was named after) was a nearly-unkillable beast of Herculanean legend. every time you cut off one head, two would grow in its place, meaning that you had to burn the stumps of the heads (cauterizing them) to prevent regrowth, eventually killing the beast (though why you couldn't just stab it through the heart, it is unknown).
- The tunnel where you can find this coin is also the tunnel where the Spartan Treasure is found. It would be easiest, therefore, to simply grab this coin on the same mission you obtain the Spartan Treasure while accompanied by GG.