This is the 100 floor underground dungoen.
Each time you jump to a new floor, you'll see a little flash on the bottom of the screen that shows what floor your on.
You can exit this place, and start where you left off. Which you should do. But you will have to start over on your current floor.
You will encounter fiends here, whether you like it or not. At floor 20,40,60, & 100 you will have to fight a boss.
At floor 40 you meet the black element, and The Bosses are tough, and meant for high leveled characters.
Also, avoid Red Dragons and The Big TonBerries.
If you get on a floor where there is a Red Dragon in the middle and it appears there is no way around it, head over to the left to create stepping stones over him.
On the floor where the Tonberry is on the left, stay to the far left and walk by him, when he's in the middle after having to jump backwards, jump carefully against the wall to step on the stone that will make the wall disappear. (If you jump too far, you will have to fight it.)
On the floor where there are little Tonberries and big ones, DO NOT run into them, walk around.