The Aetherium was a strange alternate dimension and the home of the monster Marduk as well as other animal-like energy beings. The Aetherium could be accessed at the bottom of a shaft in the main room of the Great Engine beneath the ruins of Babylon, once all four of the Engine's removable parts were installed in their proper sockets.
The Babylonians were aware that they could enter Marduk's realm if they built a Great Engine that would later be dubbed the "Infernal Machine." Under orders of their king, Nebuchadnezzar, the Babylonians constructed this device to break the barrier between their own dimension and the Aetherium, which would allow humans to physically contact Marduk. During its constructuion, the Machine was abandoned by its builders out of a popular fear; however, four human disciples fled to different corners of the Earth with the Machine's four most essential parts, in order to guard over them.
The Machine parts were reassembled in Babylon, with the reactivation of the engine in 1947, largely by Indiana Jones. He and CIA agent Sophia Hapgood were the first humans to enter the Aetherium in millennia.
The Aetherium was a bizarre place made entirely out of otherworldly alien machinery and shafts of a toxic, semi-liquid, semi-gaseous substance (now called "aether"). One could swim through the aether like water, though it poisoned any human who breathed it in for too long, resulting in sudden death after only a few minutes. The ill effects of aether, however, could be instantly cured by taking in a breath of aether-free oxygenated air.
The Aetherium pulsed everywhere with blue, blue-violet, and blue-green light and electricity. In addition to Marduk himself, Indiana Jones stumbled across the following life-forms inhabiting the Aetherium: (a) spiked, venomous, plump, insectoid creatures about the size of cats; (b) lightning-flinging, tentacled creatures that floated in the air; and (c) harmless, glowing, fish-like creatures with wings. These creatures, like much of the Aetherium, were largely blue in color.
The part of the Aetherium which Indiana Jones explored appeared to consist of a central shaft leading down through a massive palatial chamber with tunnels and other cavernous rooms built around it; however, it is not clear if this chamber made up the entire Aetherium or just the only portion of it accessible to Jones. It seems likely that this was just a small segment of the whole Aetherium that had been contained inside our own reality by the Babylonians when they had first activated the Machine.
Jones observed that the Aetherium connected to small chunks of the normal world, chamber-sized portals, all of which appeared to be places where the parts of the Infernal Machine had been originally hidden near, including small, room-sized sections of Kazakhstan, Palawan, Mexico, and Sudan. By using a device called the “Tool from Beyond” (that Jones discovered inside the Aetherium) the barrier between the Aetherium and these chambers of reality could be broken down, allowing a Jones to breathe in ordinary Earth air and thereby reverse the toxic effects of aether on the human condition. Breaking these boundaries, however, appeared to cause structural damage to the Aetherium, with the alien machinery and substance of the dimension transforming into the ordinary mud-brick ruins of Babylon when barriers between the external world and the Aetherium were dissolved. Such events also weakened the Aetherium's structure, resulting in loud cracking noises and powerful tremors.
Jones encountered Marduk, the intelligent being who ruled the Aetherium, on two occasions. Marduk, a large winged creature with a many-crested head, dismissed Jones as a mere mortal and attempted to kill him with aether blasts. Jones, however, reflected the light energy back at Marduk with a mirror, causing the monster to flee.
When Marduk later fused its body with Sophia Hapgood's, the result was a colossal bird-like humanoid. Jones again fought back and defeated the monster, releasing Hapgood's body. However, the Aetherium's structure, now weakened from Jones's breaking of the boundaries between the Aetherium and reality, suddenly began to implode. Jones and Hapgood fled as the crumbling Aetherium disappeared from existence. The entrance to it which Indy and Sophia had exited by transformed into a stone wall behind them as they re-entered reality. The weakened Marduk either perished in the collapse or was able to enter reality, but likely unable to survive in it because of its non-aetheral atmosphere. In either case, it is assumed that Marduk was consequently destroyed along with the only Earth-accessible portion of the Aetherium.