14The show takes place in the future
The landscape of Third Earth is rugged and unforgiving. There's the Bottomless Chasm, which the ThunderCats have fallen into on a number of occasions; the River of Despair, which is inhabited by a giant eel, the six-legged black widow shark, and the Whirlpool of Infinity; and Acid Lake, which
is… well… a lake filled with acid. These intimidating locales, and the prehistoric-looking monsters that inhabit them, make it seem like ThunderCats is set during primitive times. However, the title of Third Earth doesn't refer to the planets location to our sun, but instead to the time period.
From the series we know that King Arthur was alive when the planet was still referred to as First Earth. And while we may not know the events that unfolded during Second Earth, we can assume from the lack of human infrastructure and modern technology that it wasn't good. The ThunderCats arrived on a post-apocalyptic Earth, void of modern cities and advanced technology. In fact, the only technology available on Third Earth arrived via Thundera, Plun-Darr, or another outer space civilization.