The Aetherium was a strange alternate dimension. It was ruled over by an alien warlord named Marduk; whom had established himself as the chief god of the Babylonians. Marduk coveted Earth and wanted to invade and rule it; but the Aetherium was usually cut off from the normal world. So Marduk inspired the Babylonians to build the Infernal Machine; a device which would break the barrier between the worlds and allow him to invade. Luckily; the Babylonians’ ruler King Nebuchadnezzar II discovered Marduk’s true intentions and ordered the machine destroyed; and hence Marduk and the Aetherium were cut off from the Earth. The machine was reactivated and the Aetherium accessed many years later; but Indiana Jones entered it and slew Marduk.
The Aetherium was an absolutely bizarre place. It was made entirely out of otherworldly, alien machinery; and everywhere in it pulsed with blue and purple light. It had shafts and tunnels made out of a blue, gaseous substances dubbed Aether; which one could swim through like water and which the parts of the Infernal Machine were imbued with. It was inhabited by bright, glowing, dragonfly-like creatures; squid-like creatures which floated in the air; and spiked, poisonous, centipede-like creatures. The part of the Aetherium which Indiana Jones explored appeared to consist of a central shaft leading down through a massive central chamber; with tunnels and chambers built around it. However; it is not clear if this was the entire Aetherium or not.
The Aetherium connected to small chunks of the normal world; all of which appeared to be places Indiana Jones had previously visited on his adventure to collect the parts of Infernal Machine. By using a device called the “Tool from Beyond”; the barrier between the Aetherium and the chunks of ordinary reality could be broken down. This appeared to cause 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 there and the Aetherium were broken.
The Aetherium had an adverse effect on beings from the normal world who stayed there for too long. When swimming in the Aether; Indiana Jones gradually took damage (the same thing happened when he used the parts of the Infernal Machine; which were imbued with it) and although he would not continue to take damage out of the Aether; he would still stay wounded whilst in the Aetherium. By breaking the barriers to the chunks of normal reality and entering them; however; Indy would get better.
Once Indiana Jones defeated Marduk; and he and Sophia Hapgood fled; the Aetherium erased itself from existence; with the entrance to it which Indy and Sophia had exited by turning into a stone wall.