Chapter 155 : Unexpected Discovery
In Black Grill's cave, each floor was separated from the other by an impressive depth. Apart from that, the rock strata was very firm and dense making it impossible for players to blast through it and descend to the higher floors. Hence, exit gates were the only entrances to and fro each floor. Exit points[1] were special spatial nodes located within Black Grill's Cave that connected each floor to the next floor. They weren't one way gates as players could still use them to ascend to the higher floors[2].
For an unknown reason, the Exit gates repelled monsters from approaching it. Rather than repel, it would be more accurate to say that monsters consciously avoided the gates by themselves. Hence, the exit gates in each floor were informally called Safe zones. However, the reason why monsters avoided these places were unknown to the players themselves as such, players couldn't capitalize on this knowledge.