The Toflug Reef is a small mining site that can be visited early in the Gravity Rush 2 main story. It is also known as the Gravel Pit by residents of the Banga Settlement. It can be revisited later in the main story and in the Delvool Trench Mine. It also features a fighting arena.
This is actually where both Kat and Syd ended up at the end of Gravity Rush - Overture, discovered by a young girl named Cecie. Since arriving through a gravity storm, the pair have been taken in by a wandering settlement known as Banga, and forced to work the mines. The first chapter of Gravity Rush 2 actually takes place here in the Toflug Reef.
As Kat and Syd fall asleep, she is awakened by Gawan who tells them that Cecie has ran away into the gravel pit, the name given by Banga. Nevi (or Scarabs) show up in the reef itself, indicating an approaching gravity storm. Kat finds Cecie along with her missing companion, Dusty, whom Cecie had looked after since Kat's arrival.
Kat uses the gravel pit to readjust to using her powers again after receiving some disparaging comments from Misai about being a complete show-off. The fighting arena is later used when Fi challenges Kat to a game of B'oarack in order to negotiate Banga's shipping contract.
Although the Toflug Reef doesn't appear again in the main storyline, Kat does return to it in the side mission "Anchors Aweigh" to help a downtrodden man prove that there is another world beyond the gravity storms beneath Hekseville. She can return to the reef any time the player chooses to go mining.