"Henceforth no man shall set foot upon the world, and all around shall be set sentinels to ward away unwary spacecraft. We must accept that this place is lost to us forever, and is now the eternal habitation of abomination."

– from The Contagion of Ganymede

The largest moon of Jupiter and formerly a Forge World. The Adeptus Mechanicus infamously used this world as a test bed for stolen Squat technology, specifically their "Warp Fission" technology, wherein the Squats would use raw warp energy and process it as a form of power.

The Squats, being extremely more advanced than the AdMech, who have religious ceremonies to reboot the vending machines in the Forge World Lobbies after they eat your credits, are able to handle this technology. The Imperium is not.

The resulting breach caused a warp rift -- a miniature eye of terror -- to form on the planet's surface, and it has been declared off limits for all time.

It is somewhat difficult to understand how this came about, seeing as any annexed world (or abhuman species) must share all its technology with the Imperium, who must share it with the then Mechanicum (now Adeptus Mechanicus). Otherwise, it gets forcibly taken and probably ripped out of uncompliant fools' minds. The Emperor especially is all-but guaranteed to take keen, personal interest in any Warp related technology. Either reason (annexation or the Emperor's interest) is enough to ensure there is no way for the Imperium not to understand Warp fission technology. On the other hand, this may have been intentionally screwed over at the behest of the Emperor to prevent its proliferation. Better to lose a small Forge World (well Forge Moon really) than for that risk to be carried over to almost every freaking planet in the Imperium.

This episode is similar to Doom 2016's plot where the UAC (a profit-seeking megacorp) tried to tap into a dimension adjacent to literal Hell to solve Earth's energy crisis, triggering a demonic invasion in the process.

Given the time since this lore was published, the dubious canonicity of anything to do with the Homo sapiens rotundis and that more recent lore has listed the moon as the central hub of the Jovian Shipyards up to the Horus Heresy at the least, it seems quite likely that this record of events is no longer entirely accurate to the contemporary 40k setting. Or maybe the Imperium is using Warp-fission technology but since it’s result is still “plasma” (fission is plasma related i.e. mini-stars), maybe they still call the generators “plasma generators” instead of making a new name. Much like how “autogun” applies equally to caseless chemically propelled bullets as it does to bullets propelled by gravitational manipulation technology.

The Planets, Systems, Regions and Sectors of the Galaxy
Imperial Homeworlds: Holy Terra - Sacred Mars
Primarch Homeworlds: Baal - Barbarus - Caliban - Chemos - Chogoris - Colchis
Cthonia - Deliverance - Fenris - Inwit - Medusa - Nostramo
Nocturne - Nuceria - Olympia - Prospero - Ultramar
Planets: Armageddon - Bakka - Cadia - Catachan - Ganymede
Hydraphur - Krieg - Kronus - Sacris - Sanctuary 101
Scelus - Tanith - Tartarus - Vigilus - Vraks - Zayth
Daemon Worlds: Plague Planet - Sortiarius
Systems and Regions: Ghoul Stars - Halo Zone - Jericho Reach - Kaurava System
Solar System - Stygius Sector - T'au Septs - Taelus System
Types of Worlds: Agri-World - Craftworld - Daemon World - Death World - Eldar World
Forge World - Fortress World - Hive World - Tomb World
Forces of the Adeptus Mechanicus
Command: Magos (Tech Priest Dominus - Tech-Priest Manipulus)
Troops: Chrono-Gladiator - Combat Servitors - Electro Priest - Enginseer
Kataphron Battle Servitors - Myrmidon - Pteraxii - Secutarii - Serberys Raiders
Servo-Automata - Skitarii - Tech Priest - Tech Thrall - Thallax - Ursarax
Castellan-class robot - Cataphract-class robot - Colossus-class robot
Conqueror-class robot - Crusader-class robot - Scyllax-class robot
Thanatar-class robot - Vultarax stratos-automata
Walkers: Ironstrider Ballistarius - Onager Dunecrawler - Sydonian Dragoon
Vehicles: Chimera - Karacnos Assault Tank - Krios Battle Tank - Land Raider
Macrocarid Explorator - Minotaur Artillery Tank - Rhino Transport
Skorpius Hover Tank - Triaros Armoured Conveyer
Flyers: Archaeopter - Avenger Strike Fighter - Lightning Fighter - Storm Eagle
Other: Galvanic Servohauler - Ordinatus
Titans: Imperator Battle Titan - Imperial Knight - Reaver Battle Titan
Warbringer Nemesis Titan - Warhound Scout Titan - Warlord Battle Titan
Forces of the Squats
Command: Guildmaster - Living Ancestor - Squat Warlord
Troops: Brotherhood Heavy Weapons Team
Commissar - Hearthguard - Mole Mortar Team
Squat Berserkers - Squat Thunderers
Squat Trooper - Tech Priest
Vehicles: Cyclops War Machine - Colossus War Machine
Land Train - Leviathan - Rhino - Squat Bike
Squat Trike - Tunneling Transport Vehicles
Flyers &
Iron Eagle Gyrocopter
Overlord Armoured Airship
Artillery: Heavy Quad-Launcher
Thunder-Fire Cannon
Goliath Mega-Cannon