Earth

Earth (地球, Chikyū) is the planet on which One-Punch Man takes place. Although the planet largely resembles the real-world Earth, it possesses some distinctive differences.

Appearance
The Earth, unlike its real-world counterpart, possesses a single super-continent instead of multiple large landmasses. The continent has 26 cities, each identified by a letter of the modern English alphabet.

In addition to the supercontinent covering most of the planet, it possesses smaller islands and landmasses that lie in its immense ocean.

In the anime, Earth was shown to have four natural satellites instead of a single Moon, like in the manga.

History
The reason why the Earth is so radically different from its real-world counterpart is that it underwent two major "eras of change." Long ago, there were several nations that fought countless wars for land and resources. These constant wars decreased the human population significantly, so the nations of the world united to form a world government and unified the languages. This was known as the first era of change. However, the fallback from the previous wars caused several problems, such as increased environmental toxicity, rapid climate change, a rise in sea levels, and the birth of various new lifeforms. The changes made most of the planet uninhabitable, forcing humanity to migrate to the supercontinent. This was known as the second era of change.

Inhabitants
Humans are the dominant species of Earth. The planet is also home to multiple species of plants and animals. Unclassified creatures are grouped into a category of beings called Mysterious Beings. This category also includes non-human races, such as the Subterranean People, the Skyfolk, the Seafolk, and the Terror Lizard Clan.

Trivia

 * The shape of the planet's supercontinent is the same as the Saitama Prefecture in Japan. This is the prefecture that One-Punch Man author ONE was brought up in.
 * The "seven faced islands" contain potatoes.
 * The currency of the world is nearly identical to the real-world Japanese yen. The 10,000 yen note (the only banknote shown in the series) features the face of a ball-chinned man (possibly an ancestor of Agoni and the Ball-Chinned Kid) instead of Fukuzawa Yukichi.