The Lost Valley is located beneath the catacombs under the Monastery. The Lost Valley is styled after Jurassic Park by Vince, Rex, and Questor (hope I got the abbreviations right), and is recommended by the area list for levels 70 - 100. This is a pretty good estimate, although some level 70 chars will defiantly have a hard time there. There are currently no level restrictions imposed by the immortals, so anyone can visit.

Most of the mobs are non-aggressive, and most of the ones ate are aggressive, cannot see invisible. The aggressive mobs are T.rex, Velicoraptors, Allosaraus, Saber-Toothed Tiger, Giant Crocodile, and Tylosaraus. All but T.rex and the giant crocodiles are sentinel, and only Saber-toothed Tiger, velicoraptors, and Tylosaraus can see invisible.

Interesting items found here include the T.rex Crown and Raptor Rope necklaces.

T.rex crown is an item that normally is level 100 or great, but about 1 in 5 times will be level 99. It's only real use is adding stats so characters do not need to always be under the influence of giant strength and similar spells.

This is what identify reveals about this item:

Object 'T.rex crown' is type armor, extra flags hum magic nodrop.
Weight is 13, value is 2500000, level is 99.
Armor class is 19.
Worn:Takeable Head
Affects strength by 5.
Affects dexterity by 5.
Affects intelligence by 5.
Affects wisdom by 5.
Affects constitution by 5.
Affects armor class by -10.
Affects hit roll by 12.

Raptor Rope Necklaces are found on the Velicoraptor, in the Lost Valley. This armor is very good neckware for all classes, although some warrior types prefer to enchew the hp gain for the added damage roll give by wearing Square Scarfs. Raptor Rope Necklaces are typically level 90 - 94, but can be as low as level 87.

This is what identify reveals about this item:

Object 'raptor rope necklace' is type armor, extra flags dark evil magic anti-good.
Weight is 4, value is 200000, level is 92.
Armor class is 17.
Worn:Takeable Neck
Affects mana by 166.
Affects hp by 166.
Affects hit roll by 7.
Affects damage roll by 6.
Affects detect evil by 18.
Affects disarm by 2. 

The Lost Valley is a good place for characters to gain their last 15 levels or so, but there number of mobs is relatively limited, so if there are more than a 2 groups there, you would be better off seeking experience elsewhere.