First the Worldgorger Dragon needs to be in your graveyard (via Careful Study, Entomb, Buried Alive, or other such cards). Play Animate Dead, which turns into a local enchantment that pulls target creature out of any graveyard into play (meaning the Dragon). The Dragon comes into play, and summarily removes all of your other permanents from the game, including the Animate Dead and whatever lands you have in play.
When Animate Dead leaves play, however, the creature it was enchanting is destroyed. The Dragon dies. And when the Dragon leaves play, all the permanents it had removed come back into play, including your lands (untapped), and the Animate Dead. Animate Dead pulls the Dragon back out of the graveyard, its comes-into-play ability triggers – at which point you can tap your lands for mana and then everything but the Dragon is removed from the game… again. Repeat. You net an amount of mana per cycle equal to what your mana sources can produce.
If you have no way of stopping the cycle, it will continue indefinitely, ending the game in a draw. Fortunately there is the option to have a different target in the graveyard available for the Animate Dead once you have gone through the cycle enough times. Bringing out
Oona, Queen of the Fae with 500 mana in your pool is a good way to eliminate your opponent's library.