The Grathnode Inferia (グラスノスインフェリア, gurasunosuinferia?, lit. "Grathnode Inferia") is the greatest catastrophe that fell over the Planet Ar Ciel around 700 years ago, and it's the reason why the planet's surface was destroyed and left covered by the Sea of Death.
This happened in 3040 AD, when the ruling nation of Sol Cluster, El Duel, dispatched a group of Teru to silence the First Tower of Ar tonelico, so its great power coudn't continue being a threat to them. However, these Teru, better known as Necrofamilies, had other ideas: due to the great hatred they had gained towards mankind due to the total destruction of their homeland of Justine a few years prior, and upon hearing some predictions that the Ayatane Necrofamilies had recently made, they decided that it would be better to destroy Ar tonelico, and together with it, all of humanity.
Using their teleportation powers, the Ryuuhi Necrofamilies led all the others to the very core of Ar tonelico, bypassing all of the security systems and devices that were created to protect this important facility, and once there, they deliberately destroyed the power lines that sent energy from the Parameno Discs to the Grathnode Discs of the Tower. This caused the Tower to continue producing Symphonic Power without any way to release it into the atmosphere, until the point in which the systems that protected the Grathnode Discs and its surrounding power lines, the Frozen Eye, broke due to the excessive amount of power. This led to the Discs to shatter, releasing amounts of energy superior to that emitted in combination by all the nuclear power plants of Earth, and sending sonic booms that devastated the surface of the land due to causing the complete shattering of the Planet's Heart of the Land.
The energy that the Discs emitted throughout the atmosphere formed the lethal plasma sea called the Blastline, while the destruction of the Planet's Heart of the Land, and the total devastation of its surface formed the Sea of Death, which due to the grief and hatred that the Wills of the Planet began feeling towards the humans due to this, gradually extended until it covered all of Ar Ciel.
This catastrophe caused the death of millions of people around the word, including that of the famed wave researcher Eleno, and the only ones that survived were the people that managed to climb up the Towers to seek shelter. Likewise, this caused the near-total extermination of the Necrofamilies, and just a few members of the Ayatane and Homuragi clans survived because they commanded the others from the relative safety of Sol Cluster.
A flashback to this catastrophe can be seen just before the battle with Zzx_Ru in Ar tonelico Qoga: Knell of Ar Ciel, which shows the memories of sadness and hatred that the Wills of the Planet are emitting with their song, XaaaCi.