The Elemental Planes

Lively planes, all; home to many elemental creatures aligned to their element. The competition for energy and sustenance in these planes is fierce, and when these creatures get the opportunity to breach into the Material Plane they readily do so. Whilst they cannot live on our plane for long, the lure of an easy meal is difficult to resist.
Alignments to any of these planes in the City are dealt with rapidly, or else bring about rapid destruction. Still, rumours of the Merfolk harnessing the power of the plane of Water are well founded, and deep in the Crescent Dam's trench the creatures and power that pour forth from such alignments are a source of much of the Deep Merfolk's power (and the powerful enchantments they send to the surface).
Though each of the planes exists in isolation, it's speculated that all life, maybe even all of the material plane, is composed of all four elements.

Fire

A hellish place of eternal fires and roiling landscape - though solid land does seem to exist, it is perpetually aflame. Creatures from this plane seem to be the most aggressive of all elemental beings; seemingly viewing all material creatures as simple fuel.
Still a handful of Devils have made this plane their home, apparently immune to the flames, and so unassailable in their bastions, where they wait out their exile from The Hells.

Earth

A honeycomb of pockets of earth, stone, mud and more. Many plants grow here, though not the kind that people of the material plane would recognise - not plants that have ever seen a sky. Travelers have described the plane as 'superficially calm' - for although much of the plane is static, great burrowing beasts or forces churn through the earth periodically; threatening to consume an unwary traveler in a single ambivalent gulp.
Many expeditions do come here from Amaranth, to harvest elemental power, often in the form of crystals left in the wake of vast beasts. Whole families of Goblins and Dwarves dedicate their lives to carefully extracting these power sources.

Air

One of the more elusive planes, and difficult to travel to from ground level. With the invention of the Skyships, the locations of the alignments with this plane have become more clear - high in the sky. Of the few encounters crews have had with this plane, the otherworldly creatures from that place seem skittish and flighty, though not unwilling to try and attack the unwary - causing damage to a couple of Skyships early last year.

Water

Only the Merfolk tend to be familiar with this plane, and even then only on a fleeting basis. Though not dissimilar to the dark depths of the deep ocean, the plane itself is far more disorienting, losing a sense of up or down, or a sun or moon overhead. Accordingly, the Merfolk tend to try and entice the creatures of this plane out into our own before taking their power or service.