The Sea of Death (死の雲海, shi no unkai?, lit. "Cloud Sea of Death") is a massive cloud formation that covers pretty much the entirety of Planet Ar Ciel. The only areas that haven't been covered by it are the middle-to-upper area of the three Towers of Ar tonelico and their surrounding lands. The clouds themselves have a lethal poison, so living inside them is somewhat impossible.
The Sea of Death was formed in 3040 AD during the Grathnode Inferia catastrophe, in which the First Tower of Ar tonelico was sabotaged and its power regulator systems were destroyed. This caused the Grathnode Discs, the power release system for the Tower, to overload and shatter from the overwhelming amount of energy, releasing enormous quantities of energy that destroyed the Planet's Heart of the Land and ripped the land apart. All the dust, volcanic gases and smoke that this event produced gave birth to the Sea of Death.
However, this enormous toxic cloud would have lasted only for a few months if it wasn't for other factor: the Wills of Planet Ar Ciel. The Planet Herself was filled with sadness and hatred for the humans due to the enormous damage they had caused to Her, and thus, as an expression of these feelings, she manipulated the toxic clouds to make them cover Her surface. The Sea of Death gradually expanded its reach and height, until the point in which it covered a 99.3% of the Planet's surface, and left it uninhabitable for 700 years. Everything that fell into the Sea, or that tried to go inside it for any reason, disappeared and was never seen again.
The Sea of Death was finally cleared in 3775 AD, when a party led by the Reyvateil Origin Tyria finally managed to carry out the Planet Regeneration Project, endowing Ar Ciel with a new Heart and gaining the forgiveness from all its Wills. This allowed the surface to be seen again for the first time in 700 years, and allowed everyone to live in the ground again after the long time in which they had to cling to the Towers for their survival.
While the Sea of Death itself may seem as just a massive mass of clouds or mist, it is actually something far worse. As an expression of Ar Ciel's grief and hatred, this massive toxic mass is not only poisonous: it also destroys and disappears from existence anything that happens to be covered by or falls inside it, which is why no one has ever returned upon venturing into this place.
Metafalss' notorious researcher Infel and several researchers from the Archia Think Tank in Sol Cluster, among them the expert airship pilot Luke, all tried to perform all sorts of research on the Sea of Death to find countermeasures to it, but they all ended in failure: on one hand, Luke lost his life after heading into the Sea. Infel, on the other hand had the idea of sending people inside of air-locked chambers suspended by chains, but once the chamber hit the clouds, it simply vanished away, with the chains seemingly broken by something.
This is also because the Sea of Death is the natural habitat to the Antibodies: the monsters that Ar Ciel created in order to regain what few energy She could from the currently living creatures on Her surface, in a desperate effort to save Herself. Therefore, even if it was hypothetically possible to enter the Sea of Death unscathed, most likely the one entering there would be swarmed and devoured by the Antibodies.
However, Ar Ciel wasn't completely ruthless about this: with the exception of the Third Tower, because of its Moocheriel facility, the Towers of Ar tonelico wouldn't be attacked or destroyed by the Antibodies or the effects of the Sea of Death. In fact, the only natural phenomena that would affect the Towers would be natural decay.
Finally, the only reason the XP Shell and its occupants aren't subject to the Sea's lethal effects is because of the tremendous speed at which the Shell is shot: given it has an acceleration of 200 times Ar Ciel's normal gravity, the Sea and the Antibodies wouldn't have time to do anything to it until it had landed into the surface.