The Garden of Eternity is an area of statues, trees and gardens on the outskirts of the Land of the Lost somewhere off the river flowing from the center of it. It's exact location is unknown, but it may be at an opposite end of the land from The Lost City.
The Garden of Eternity was created by Medusa, a denizen from from Ancient Greece who may have been the famous Gorgon of myth. She created the garden out of the recurring victims trapped in the Land of the Lost, turning them to stone as the statues in her garden under her powers. Among her victims seems to be Jefferson Davis Collie III posed with his cannon. It is also adorned with Greek pillars and a portico with her mirror; the origins of these artifacts are unknown, but it may be they were accidentally transported with her. To protect her, she employed a series of almost lethal strangler vines to follow her bidding. Again, the origins of these vines are unknown. They may be indigenous to the Land of the Lost, having gained allegiance to her through her care of them.
Eventually, Medusa wanted to add Holly Marshall and her family to her garden. In doing so, she kept Holly as a guest against her will, but Jack Marshall eventually deduced who Medusa was and defeated her by turning her mirror against her, turning her into a statue.
It is unknown as to what happened to the Garden of Eternity after Medusa was petrified, but it is likely it was eventually reclaimed by the wilderness.