The Eastern Sea is an extensive body of water that extends from the eastern coast of the Earth Kingdom to the northeastern shores of the South Pole. A small waterway connects this sea to East Lake. A great number of islands are located in the Eastern Sea, including the Eastern Air Temple's archipelago.
The sea acts as lifeline to many adjacent communities who live from fishing and trading; regardless, the region was never particularly wealthy.
After its unification, the Earth Kingdom began to claim the Eastern Sea as part of its territorial waters. At some point, however, the Fire Nation's navy intervened in the sea. The 22nd Earth King eventually put an end to the Fire Nation's presence by prohibiting the Fire Navy from flying military colors in the Eastern Sea. In addition, the Earth Kingdom took possession of the Chuje Islands, although they were claimed by the Southern Water Tribe.
By the 4th and 3rd centuries BG, the Eastern Sea became dominated by a great number of pirate groups such as the Fifth Nation and the Fade-Red Devils. The pirates regularly attacked trade ships and fishing villages along the sea's shores. Many communities reacted by creating warning systems that included watchtowers, drums, and bells. Regardless, these systems provided little actual defense, and locals lived in constant fear of raids. As a result of the relative poverty of the region, the local pirate groups often fought each other in brutal turf wars. The Earth Kingdom's navy attempted to counter the growth of piracy to no avail. At some point between 305 and 296 BG, the Fifth Nation pirates utterly defeated the Earth Kingdom's navy in battle and killed the last admiral of Ba Sing Se. Thereafter, the Eastern Sea became completely lawless.
The Eastern Sea remained at peace until the Hundred Year War commenced centuries later. Ships from the Fire Nation continually bombarded the area, attempting to break through the Earth Kingdom's defenses and reach Ba Sing Se. In 97 AG, the 41st Division of the Fire Nation military arrived at the Eastern Sea, though General Bujing suggested during a war meeting to leave the sea and head north to fight a powerful battalion of earthbenders near Ba Sing Se.
Later that year, Chief Hakoda and his troops from the Southern Water Tribe completed their journey to the Eastern Sea, where they stationed themselves at Chameleon Bay in order to stop any Fire Nation advance toward Ba Sing Se via the water. Here, Hakoda invented the tangle mine, a weapon used to combat invader ships. Years later, during one of his nightly patrols over the water on a cutter sailing ship, Hakoda spotted Appa flying overhead, who was attempting to find Aang.
During the spring of 100 AG, Hakoda and his troops' base camp were stationed on the northern shores of the sea, on a beach, where the chief later reunited with his son, Sokka. Shortly after their reunion, the Water Tribe warriors prepared to fend off an invasion of Fire Nation ships. However, before Sokka could leave with them, Aang landed on the shore to pick up the warrior due to the former's vision of Katara in trouble.