Question about the abyss

Hi guys, I'm currently watching the anime and I'm on episode 6. I have a question about the abyss but I can't answer myself cause I'm not able to see again previous episodes: how's divided the abyss and how deep it is? I remember them stating that the limit reached by humanity is 600m, but Riko and Reg reached 1800m with unbelievable ease. I remember that the abyss is divided into different "levels", but I am surely missing something!