The Blastline was formed in 3040 AD, due to the Grathnode Inferia catastrophe. The shattering of the Grathnode Discs of the First Tower of Ar tonelico that caused the catastrophe also released massive amounts of plasma into the atmosphere, which created this sea of plasma that encircles the entirety of the Planet, pretty much sealing off all access to the outer reaches of the atmosphere to all airships built at the time.