Lake Lerna

Lake Lerna (レルナ湖 Reruna Ko) is a lake in the north pole of the Overworld. Fort Lerna, commanded by Codename Hydra, is located here.

Geography
Lake Lerna is surrounded by small mountains, with entrances in the cardinal directions. In the middle of the lake is Lerna Falls (レルナ滝 Reruna Taki), which falls into an abyss that leads to the Underworld. This is the only known entrance into the parallel realm.

The dimensions are distorted near the lake, which makes gravity stronger. As a result, time flows slower, and the soldiers stationed here are chronologically older than they are biologically.

Trivia

 * Lake Lerna is based on the Lernaean Lake, which, in Greek mythology, is the entrance to the Underworld.