The Elemental Planes are locations featured in Masters of the Elements and mentioned many times through the game. The five Planes are each ruled by an Elemental Lord.
The Elemental Plane of Air
The Elemental Plane of Air is a Plane where the Elemental Lord of Air is ruling. The land itself is covered in clouds (oddly, vegetation grows there as if there was actual land underneath the clouds). The inhabitants there are mostly Air Elementals and Genies.
In Heroes Chronicles, the Wizard Tarnum is required to capture the Conflux to master Air Magic.
The Elemental Plane of Water
The Elemental Plane of Water is a Plane where the Elemental Lord of Water is ruling. The land itself is a swamp where usually a hero needs a boat to travel. The inhabitants there are mostly Water Elementals, along with creatures from Tatalia.
In Heroes Chronicles, the Wizard Tarnum is required to capture all towns from the Elemental Plane of Water to gain mastery over Water Magic.
The Elemental Plane of Earth
The Elemental Plane of Earth is a Plane where the Elemental Lord of Earth is ruling. The land itself is covered by rocks and mud. It seems to have a stronger gravity than the other Elemental Planes. The inhabitants there are mostly Earth Elementals, along with creatures similar from Nighon land. It is mentioned that a band of Nagas also lived there.
In Heroes Chronicles, the Wizard Tarnum is required to defeat the Nighon creatures so he can master the Earth Magic.
The Elemental Plane of Fire
The Elemental Plane of Fire is a Plane where the Elemental Lord of Fire is ruling. The land itself is covered in flames which do not burn living beings. Sometimes, a hero can encounter a Pandora's Box. The inhabitants there are mostly Fire Elementals, along with Demons.
In Heroes Chronicles, the Wizard Tarnum is required to capture the Conflux to free the enslaved phoenixes.
The Elemental Plane of Magic
The Elemental Plane of Magic is populated by Psychic elementals, led by magic elementals. There is little that is fixed in the Plane of Magic - rocks can turn into birds as one walks past them.