Terra

Terra, also called "Holy Terra" or, in the most ancient records, "Earth," is the throne world of the Imperium of Man and the original homeworld of Mankind and of the Emperor. It is the most sacred and revered place in all the million worlds that comprise the Imperium. Billions of pilgrims from throughout the Imperium flock to Terra -- even the barren and contaminated soil that these pious folk tread upon when they reach humanity's homeworld is considered holy by believers in the Imperial Creed. Terra is effectively a globe-straddling temple dedicated to the worship of the Emperor of Mankind. The planet is home to the primary headquarters of many important Adepta of the Imperium since it serves as the capital world of the Emperor's realm. There is a great fear of Space Marines amongst the people who dwell there, due to events dating back to the Horus Heresy, when much of the planet was leveled and terrorised by the actions of the Traitor Legions. Before the Great Crusade, Terra was plagued by a long age of civil war and anarchy known as the Age of Strife, which only came to an end when the Emperor personally waged the Unification Wars to reunite all the Terran factions under his own enlightened leadership. Terra is perhaps the most massive Hive World within the Imperium, with a population of over one hundred trillion human beings. The bulk of Terra's massive population is divided between the upper class that includes the Imperial nobility and Adepts of the Administratum and the lower classes who serve as basic labourers. The upper class is comprised of the official servants of the Emperor and his Imperium, including Imperial officials, Ecclesiarchy clergy, the aristocratic families of the Navis Nobilite, military officers, scribes, and bureaucrats. The teeming masses who comprise the Terran lower classes are far less-privileged, and many of them are no better than serfs or thralls who toil in the world's countless manufactorums, providing the infrastructure that keeps the heart of the Imperium's bureaucracy functioning. Terra has one moon, Luna, which is an inhabited and highly populous Civilised World of the Imperium in its own right.

Old Earth
The Age of Terra, also called the Age of Progress, is the name given by Imperial scholars to the time from the dawn of Mankind's civilisation before the Imperial Calendar was instituted in the 1st Millennium AD to the founding of the Imperium of Man in the 31st Millennium. Cities such as Atlantys and Nova Yourk are cited as being the most legendary and ancient cities of Old Earth. Nations now known in the oldest and most incomplete fragments of Imperial records only as Jermani, Merica, Britania, and Bania are said to have both prospered, competed and wilted during this legendary time when humanity was climbing its way towards the stars.

Geography
Terra in the 41st Millennium is an Imperial Hive World, the largest in the galaxy. The Earth was stripped of all forms of natural resources many millennia ago; its soil is utterly barren and its atmosphere is now a fog of industrial pollution. Massive, labyrinthine edifices of state sprawl across the vast majority of the surface. What remained of Terra's oceans after the thermonuclear wars that scarred the planet during the Age of Strife long ago boiled away due to the immense heat produced by the billions of people who have been compressed into the world's limited living space. All liquid water to meet the Terran population's needs is now delivered from orbit by freighters who take large ice-bearing comets from the outer Solar System and bring them into Terran orbit to be melted down and dispersed to the population. Many mountain ranges have been leveled - perhaps all but the Himalayas, which remain untouched due to the genetic engineering laboratories said to lie beneath them where the Emperor created the Primarchs and the first Space Marines after the Unification Wars more than ten millennia ago. The chambers of the Astronomican also course throughout the whole mountain range. Despite being devastated during the Horus Heresy and by the terrible wars fought across the surface of the world by the nations of techno-barbarians during the long Age of Strife, Terra is still the most vastly-populated and built-up hive world in the Imperium. Beneath countless layers and millennia of urban accretion, catacombs hold older cultures radically different from the surface ones. Much of the population of Terra exists in the most terrible squalor, their greatest hope that one of their offspring might be accepted into the Adeptus Terra, the Priesthood of Earth and bureaucracy of the Imperium, as a Menial, an Adept of the lowliest sort. A square meter of land on Terra costs more than a palace on any other hive world; only the most wealthy citizens can afford to own a small section of land. Terra's entire surface, with the exception of the Antarctic region, is covered in labyrinthine edifices of state, including the Imperial Palace, the Ecclesiarchal Palace of the Adeptus Ministorum and the many departmental headquarters of the Adeptus Terra, the Imperial government. The headquarters of the Imperial Inquisition on Terra lies beneath the ice caps of the southern polar region of Antarctica and is one of the most secure installations in the entire Imperium.

Oceans of Terra
By the time of the Unification Wars there was only one remaining ocean on Terra -- the so-called Great Ocean -- which was the ancient Pacific Ocean. The size of the Pacific was vastly smaller than it once was, due to the vaporisation of a massive stretch of the ocean during the nuclear wars that had consumed much of Old Earth in the lost years of the Age of Strife. This missing portion of the Pacific stretched from the Marianas Trench down past Australia and all the way to the frozen continent of Antarctica. The Atlantic Ocean had also been largely vaporised as the great body of water that had once been near Europe was now mostly gone, particularly to the south of what had been the Iberian Peninsula. Other, smaller, bodies of water may still have remained during this time, as a great body of water is recorded to have been present in the region of Eurasia where the Caspian Sea had lain in ages past. These bodies of water still existed at the start of the Horus Heresy in the early 31st Millennium, but by the time of the 41st Millennium, Terra had no free-standing bodies of water, having become a true ecumenopolis, a world completely covered by the dense urban areas of massive hive cities. Within these human hives toiled the tens of billions of Adeptus Terra bureaucrats and serfs required to keep the decaying heart of the Imperium of Man pumping.

Locations
Terra is the heart of the Imperium, and of many of its most important organizations. Most importantly, it is the home world and resting place of the Emperor of Mankind. The most important places and organizations based on Terra include:
 * Imperial Palace - One of the largest structures on Terra, the Imperial Palace is more like a sprawling hive city than a single edifice. It covers the better part of the Northern Hemisphere, and is guarded by the Adeptus Custodes, who rarely leave the Palace. It is the heart of the Administratum as well as the home of the Sanctum Imperialis, the great throne room of the Emperor and the location of the Golden Throne. The Palace is divided between the Inner and Outer Palace. It is described as "an endless, black hive of forbidden technology and subterranean passages delving deep within the bowels of the planet."
 * Eternity Gate - The largest entryway into the Inner Imperial Palace's Sanctum Imperialis. A mile-long passage leads to the Eternity Gate, lined with the banners of thousands of the greatest and long-dead Imperial heroes, including Lord Commander Solar Macharius. It constitutes the end of the galactic pilgrimage trail, and is itself the most important pilgrimage site in the entire Imperium, for almost no one is ever allowed entrance through the Eternity Gate into the Sanctum Imperialis.
 * Sanctum Imperialis - The throne room and heart of the Imperial Palace. It is the massive chamber housing the Golden Throne and the Emperor's body. It is guarded by a select group of three hundred Adeptus Custodes, the elite among the elite.
 * Adeptus Terra - The Adeptus Terra, known commonly as the Priesthood of Earth, is the actual government of the Imperium of Man, consisting of countless different organizations and the departments within them. Besides the vast, destitute population of non-adepts that makes up the majority of the planet's populace, much of the rest of Terra's population are members of the Adeptus Terra and of these the largest percentage are adepts of the Administratum.
 * Administratum - The Administratum is the administrative and bureaucratic division of the Adeptus Terra, consisting of untold billions of clerks, scribes, bureaucrats and administrative staff constantly working to manage the Imperium at every level, from assembling war fleets to levying taxes and tithes. It is the largest of the departments comprising the Adeptus Terra, and ten billion Administratum adepts work in the Imperial Palace alone.
 * The Forbidden Fortress - Built throughout the mountain range of the Himalayas. A single peak is carved to form the Chamber of the Astronomican, where ten thousand psykers continuously power the Astronomican beacon with their own life energies. The "psychic light" of the beacon reaches thousands of light years across the Warp. The Astronomican is utilized by the mutant psykers known as Navigators to guide interstellar ships through the otherwise trackless chaos of the Warp. Without it, long-distance Warp travel would be impossible and the Imperium of Man would disintegrate.
 * Office of the Inquisition - The Inquisition's headquarters on Terra is a sprawling and highly secure complex built beneath the south polar ice cap of Antarctica.
 * Navigator's Quarter - The headquarters of the families of the Navis Nobilite (the Navigators). It is said to be on a massive island.
 * Hall of Judgment - The headquarters of the Adeptus Arbites, the chief law enforcement organization of the Imperium.
 * Officio Assassinorum - Although its agents operate throughout the Imperium, this dark organization is based on Terra. Its School of Assassins and most of the temples that comprise the Officio are said to be located on Terra, although their actual locations are closely guarded secrets.
 * Adeptus Astra Telepathica - The Adeptus Astra Telepathica is the organization responsible for recruiting and training officially-sanctioned psykers for service to the Imperium. The infamous Black Ships of the Adeptus Astra Telepathica travel throughout the Imperium, gathering their tithe of psykers and returning with them to Terra. The psykers are analysed and sorted according to their psychic power and strength of mind in resisting the temptations of Chaos. The novitiates are trained at the Scholastia Psykana for five years before being sent for further training elsewhere. Some of these sanctioned psykers become Astropaths, specialist psykers who are trained to telepathically send and receive messages over interstellar distances (no other form of instantaneous interstellar communication exists). Others are requisitioned by the Adeptus Astronomica for service in the Astronomican's choir. The most powerful, those who demonstrate a strong enough mental character to resist daemonic possession on their own, often enter the Inquisition for training as Acolytes, with some eventually becoming Inquisitors themselves. Those psykers deemed too weak or dangerous to live become sacrifices to the Emperor; their souls are "fed" into the arcane technology of the Golden Throne so that the Emperor may continue to live and guide the Astronomicon that allows the Imperium to function as an interstellar civilization. Only psykers that have been trained at the Scholastia Psykana are legally sanctioned by the Imperium; all others are considered illegal, often branded as "witches," and are at risk of being summarily executed as heretics by the Inquisition's Ordo Hereticus and other organs of official Imperial power.
 * The Ecclesiarchal Palace - The headquarters of the Ecclesiarchy (Adeptus Ministorum), the Ecclesiarchal Palace sprawls over almost the entirety of Terra's southern continent of Australia. It is also home to one of the two major convent-fortresses (the Sanctorum) of the Adepta Sororitas (the Sisters of Battle) on the Imperium's throneworld.
 * The Senatorum Imperialis of the High Lords of Terra - This immense building houses the council of the twelve highest-ranking political officials in the Imperium, the High Lords of Terra, who rule in the Emperor's name as he no longer can, though his mind is believed to remain in the Warp, guiding and protecting humanity.

The Golden Throne
Terra is the resting place of the immortal Emperor of Mankind, where he has sat on the life-preserving cybernetic Golden Throne, neither alive nor truly dead, for ten thousand years. He and the Imperial Palace are guarded by the elite Adeptus Custodes. Among them is a select inner corps of three hundred who never leave his side, known as the Companions.

The Astronomican
The Astronomican is a psychic beacon located on Terra, and is a vital navigation aide to the mutant Navigators who pilot the Imperium's spacegoing vessels through the chaotic eddies of the Warp. Without its telepathic guidance, faster-than-light travel through the Warp would be next to impossible and the Imperium of Man would collapse economically, militarily and politically.

Luna
Luna (the Moon) has also been colonized and is home to immense planetary defense lasers charged with protecting Terra from invasion. These defenses inflicted savage losses on the invading Traitor Legion fleets of the Warmaster Horus, during the latter stages of the Horus Heresy. Luna served as the greatest shipyard and naval base in the Imperium of Man at the time of the Great Crusade and the Horus Heresy. Luna also gives its name to a common class of Imperial Navy cruiser, the Luna-class.

Pilgrimage
Billions of Imperial Cult pilgrims flock to the planet every day, eager for a glimpse of the Imperial Palace or one of the untold number of gargantuan Imperial cathedrals of the Ecclesiarchy. In spite of the fact that millions of pilgrims are accepted daily, many more are kept waiting. Such is the scale of the Imperium of Man in the 41st Millennium that many of these pilgrims' journeys were started by their ancestors and only centuries later will a member of the family actually complete the pilgrimage to Terra. Many will set out hopeful and never come close to their goal. Those who make it are said to never return, crushed to death by their fellow pilgrims, executed by the Adeptus Arbites for straying into restricted areas, or perhaps killed by the crazed and destitute hive city citizens, violent gangers, etc.