Chrono Trigger/Lost Sanctum

This is a small area accessible in 65,000,000 BC and 600 AD. It is located at a northern point near Ozzie's fort in 600 AD, and near the Sun Shrine in 65,000,000 BC.

65,000,000 BC
If you go to the Lost Sanctum during 600 AD, you'll find it overrun with monsters. To get around this, simply go there during 65,000,000 BC and you'll find that same cave deserted. Head northeast and exit the cave to a northern forest. You'll spot a group of monsters there, and you're team will decide to vanquish them. I'm simply starting this page so I won't go into detail about where and what all the treasures are, but if you look at the map on the bottom screen, you'll find the little hidden areas easily enough. After you've vanquished all the monsters, head south to the cave and you'll find a reptite. Apparently, they weren't all killed by Lavos after all. You'll learn that the Reptites have been struggling to survive against those very same monsters (Pushovers...) and that you've given them freedom. You can agree to do various missions for the Reptites for a compensation of some kind. There is also a lower forest to explore, but someone else can document that.

600 AD
Head north from Ozzie's fort to find the Lost Sanctum. Upon entrance, you'll see that the monsters that were there before are now gone. As an added bonus, the Reptites of this period are giving reward missions to your "Legendary Heroes (Or simply hairless apes)". Your first mission is to go clear that same forest of monsters. Mostly, you'll face Imps. New treasures have popped up to, so look into those. Afterwards, talk o the Reptite elder for a Megalixir.