Each wheel is at least the size of an island, meaning a few miles across, to the largest cogs being thousands of miles across. Each of the interlocking wheels in Mechanus are connected to one another, though some may be rotated so that they are vertical compared to another cog, though gravity is subjective to which cog you are currently on, so the cogs don't interfere with each other's inhabitants. This plane is filled with an unlimited number of these cogs; the spaces in between the disks is a void of nothingness that extends out infinitely in all directions.

An Outsider’s Perspective

First arriving on Mechanus, one might notice that it is neither too hot nor too cold, but rather it has equal measures of each. While that is a strange thing to notice, it is what sets this plane apart from all the other planes of existence. Mechanus is a plane of law, order and equal measures of everything. It has exactly 12 hours of light that is exuded from the void surrounding the cogs and exactly 12 hours of darkness. Every cog is connected and they all move in unison. All law is reflected here and this plane, and its interconnected wheels, will always spin as one.

Traveling across Mechanus might seem to be a lot like the Material Plane depending on which wheel you first appear on. Every cog starts out as a blank piece of metal ore and stone with only the size differencing it from another disk. Deities and mortals with sufficient magic can shape a cog to their will, morphing the cogs into their ideal, lawful home. Some disks have been covered in farms and forests, while others are home to massive factories for the Inevitables.

A Native’s Perspective

Mechanus is a plane of order and law, and those who make their home here reflect that. While the plane looks straightforward, there are hidden subtleties throughout this plane. From understood codes of decorum to the sinister contracts of devils, this plane can be a dangerous place for those who begin making deals they do not understand.

The inhabitants of the plane are first and foremost the Modrons who care for and make repairs on the massive disks that hold all of Mechanus. They are the strange clockwork beings that exist in a rigid hierarchy with the lowest of their kind, the monodrone, carrying out orders with no thought to what they are. The Modrons occupy a portion of the plane known as Regulus, where their godlike deity, Primus, oversees their existence.

All cogs start out with no matter but the stone and ore they are made of, but outsiders from the plane bring with them vegetation, soil and more to Mechanus. The communities are focused on creating perfection by removing passions, illusion, pain, and unity. From the lawful Formians who have built hives across a swath of Mechanus, to the Inevitable constructs who have built massive factories devoted to the creation of Inevitables, all creatures are focused not on the individual but the whole of their species.

Atmosphere

Mechanus has an atmosphere that allows any mortal to exist on this plane and, despite the Inevitable’s factories that produce smog and soot, many never notice any pollution. The plane is infinite, and as the plane is filled with cogs, the cogs are infinite in number as well. They all spin together, with many assuming that the interconnected disks are linked to the whole multiverse if the wheels were to ever stop spinning than the multiverse would come to an end.

Traits

A plane of interconnected disks, Mechanus is filled with the orderly processions of constructs, laws, and ideas. Chaos is not anything that exists on Mechanus, and if it ever does prop its head out, the Modrons and other inhabitants of the plane do their best to destroy it.

Travel to the Plane

Traveling to Mechanus can easily be done via magical teleportation spells like Plane Shift or by finding a portal. Traveling through a portal is easiest from the linked Outer Planes of Arcadia or Acheron to Mechanus as these portals never change their locations and are constant.

Inside of Mechanus, whenever a cog finishes a full rotation a portal will appear in its center, linking it to another location somewhere in the multiverse. These portals last for up to a day, and each portal takes on the appearance of an ephemeral green cog, slowly rotating in the exact center of the wheel.

Traversing the Plane

Traveling across Mechanus is pretty easy as the cogs, unless an outside presence has worked on them, are completely barren and devoid of any obstruction that would mar its perfectly flat face. To move from one cog to another is as simple as walking across the spinning gears from one wheel to the other, though for newcomers it might be a bit unnerving and it isn’t unheard of for visitors to stumble across the gap and become crushed and pulverized by the gears.

Another option for traversing the plane is just to fly between the cogs, as the void that surrounds all of the cogs is the same atmosphere as found on the Material Plane. This makes it easy for creatures with wings or the ability to cast spells to move across the plane, and they are in no risk of falling between gears and the Modrons don’t have to come and clean up their mess once they die from being crushed.

Despite that many of the wheels are not yet colonized, there are several claimed wheels that heavily discourage visitors. The hives of Formians, with their focus on colonies and expansion, see outsiders as threats to their queen and fight to defend her and their hive-cities. There are also several factions devoted to order and law, like the Fraternity of Orders, who are also known as Guvners and who are devoted to upholding any and all laws they are aware of and exploiting any loopholes.

Illusions and Wild Magic

Illusions and sources of wild magic only serve to create chaos, and on Mechanus that is unthinkable. When illusions are produced on Mechanus, if they work at all, look like thin, smokey wisps that fool no one. Wild magic, on the other hand, is tightly regimented and forced into some sort of order, with those who practice it far removed from their power and barely able to cast the weakest of spells. Several wild magic users have been forced to undergo a tribunal on Mechanus, and are given a death sentence for their practice of it.

The Great Modron March

Every 17 Grand Cycles, the Modrons perform a massive march through all of the planes. A single Grand Cycle is the time it takes the largest gear in Mechanus to rotate once, which is what the inhabitants of Mechanus view as a year. Every 17 Grand Cycles, that is roughly 289 years for those who live in places like Sigil or the Material Planes, a march begins.

Once 17 Grand Cycles has passed, the Modrons, as commanded by Primus, form into a massive march and begin their tour of the planes. They travel in and out of the Outlands and their travels take them through each of the Outer Planes, starting with Arcadia and then journeying to Mount Celestia, then Bytopia, Elysium, the Beastlands, Arborea, Ysgard, Limbo, Bedlam, Pandemonium, the Abyss, Carceri, Gray Waste, Gehenna, Baator, Acheron, and finally back into Mechanus.

No one is sure what the Modrons do on this march, but it is no secret that thousands of Modrons begin the march, and only a few dozen ever return to Mechanus at the end. It is a deadly march and the marching Modrons never take the same path twice. The march was only started early once when it was claimed that the deity of Modrons, Primus, was killed and the Modrons controlled by a dark demon god, who was long thought to be dead.