There's a chasm, a great rend in the earth, in these parts. A scar left from High Summoner Gandof's bitter battle with Sin, four hundred years past.
Lord Gandof is one of the five summoners to have defeated Sin using the Final Aeon in the backstory of Final Fantasy X, earning him the title of high summoner. Statues of him have been erected in the temples of Yevon.
This gorge is called the Scar. They say that when the high summoner cornered Sin, Sin tore the earth here asunder with the last of its strength. The high summoner and Sin struck each other down at the gorge's bottom. Lord Gandof's sacrifice brought us the Calm.
Gandof was the one who sealed up the Qactuars roaming about the Thunder Plains in stone, as he believed that defeating Sin was not the only thing summoners could do to help Spira. During his final battle with Sin around four hundred years ago, Gandof left his mark with the formation of a huge rift in the Calm Lands that became known as "the Scar". This isolated the Remiem Temple, and started the Second Calm. Gandof's sacrifice lent credence to Yevon's teachings, and doubts about the temples subsided.
The Calm was short-lived and Yu Yevon rebuilt Sin which started its rampage again as the third Sin appeared publicly with Gandof's Final Aeon as its core. Gandof was posthumously titled "high summoner", which would from then on be bestowed on all who had defeated Sin. Statues him were placed in Spira's temples and his story was told to all Spirans as an example of honor and sacrifice to be followed.