Chrono Trigger/Dimensional Vortex

In the Nintendo DS version, one exclusive area exists called the Dimensional Vortex. The Dimensional Vortex is a dungeon with that can only be accessed after beating the game at least once. When you return to your saved game, three new Gates will have appeared in separate time periods (12,000 B.C., 1000 A.D., and 2300 A.D.).

Like other dungeons in the game, the Dimensional Vortex varies depending on which time period you're in when you pass through the gate. Each time period specifies the reward from completing the dungeon: a permanent upgrade to one of three characters: Marle (the past), Chrono (the present), or Lucca (the future).

After completing all three different versions of the dungeon, a final, new, boss encounter is unlocked (as well as a new ending).

Dungeon entrance
Passing through any of the three Gates will take you to this area. You'll have to fight (or sneak) your way through several random maps from the game's locations before moving on.

After completing this first part of the dungeon, you'll enter into a room with a save point. Two additional paths lie in the room, one that takes you out of the Dimensional Vortex, and one that leads to the Twilight Grotto. This final area depends on which gate you entered through, and will also determine which boss you fight and which character receives a permanent stat boost.

12,000 B.C. dungeon
A large icy mountain. Special enemy is a Heckran like Snowbeast. at the end, you'll get to the Twilight Grotto where you'll fight a special boss if you haven't already done so. You'll move on to what I like to call the Twilit pool and Marle will say something is calling her in. Move on to fight Alabaster Shade, a Marle look-a-like.

1000 A.D. dungeon
A large volcano dungeon. At the end, you'll fight the special boss if you haven't already. At the Twilit pool, you'll need to have rescued Chrono for this shade.

2300 A.D. dungeon
Another freakin' dome place. As usual there is relentless security. If you make it to the Twilit pool, you'll get to fight Lucca's shade, the Crimson Shade.

Twilight Grotto
You'll enter a greenish room. When you make it to a large pond in the center of the room, You'll run into Dalton. Looks like that loser wasn't dead after all. Boss fight!

Dalton will be pretty pissed that you beat him again, so he'll say something about going to Porre to raise an army, but I went to Porre in both time periods, and Dalton remains to be seen.