Ganymede

Ganymede before the AdMech fucked it up to an unimaginable proportion.

"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 far 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 most certainly is not. (no not even Belisarius Cawl)

The resulting breach caused a warp rift -- a miniature eye of terror -- to form on the planet's surface, and it has had been declared off limits for all time...until some guy went and closed the rift, and killed every single living thing in the facility...friend or foe alike.

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. Not to mention when it was eventually cleansed every single techpriest on the moon was brutally murdered. probably on orders from the Emperor.

A similar incident occurred on mars during the dark age of technology where a profit-seeking mega-corporation named Union Aerospace tried to tap into the warp to solve Earth's energy crisis, triggering a demonic incursion in the process, this then lead to the corporation being turned into a chaos cult, and a second much larger Possibly C'tan lead demonic incursion on terra. both were dealt with by the same guy who eventually cleansed Ganymede much later.

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 its 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.

Fun fact, this moon entirely exists in real life and was named after a Greek mythological man who was the most beautiful mortal ever so Zeus took him away to Olympus serve as his cupbearer and other stuff (it wasn't just the ladies Zeus banged without constraint, their resulting pregnancies just made them more famous).