Holon

Holon is a region that is presumably north to Kanto, considering that Holon town seems to be close to Eidolon Forest. It is thus far the only region consisting of δdelta species pokemon.

Story

Holon's first formal introduction was in Ex Delta Species. The location in that set seemed to be a city where scientists were very anxious to find Mew, presumably via the city tower's electromagnetic waves. However, somehow or another the electromagnetic waves caused the pokemon nearby to turn into different types, often the final form bearing steel as a type. The affected pokemon came to be known as delta species.

These electromagnetic waves seemed to travel very far; eventually pokemon far off would not be part-steel, presumably due to delta species eventually breeding naturally.

Also, in Ex Crystal Guardians, it is unclear whether the electromagnetic radiation or the crystals caused pokemon to become delta species.

Geography

In Ex Delta Species, the main landscape was city walkways with the Holon tower spewing out electromagnetic radiation in the background. The city also seemed to have canals, seemingly with Aquapolis in mind.

In Ex Holon Phantoms, the main landscape is a lake, surrounded by marshes. Strangely enough, this lake seems to have a shining gyarados just like the Lake of Rage.

In Ex Crystal Guardians, the main landscape consists of islands bearing strange crystals, which seem to change pokemon to delta species. It is unknown whether the electromagnetic radiation is still a factor of delta species pokemon in the crystal Islands.

In Ex Dragon Frontiers, the last set thus far containing delta species pokemon, the main landscape seems to be mountainous area so high up that often only clouds are seen down below. It is likely that the mountainous area is actually a collection of island mountains.