Laurentian Abyss

The Laurentian Abyss is the second deepest deepest part of the Earth's oceans, where all the defeated Decepticons except Starscream were thrown into. It was hoped that the high pressure and temperature of the Abyss would destroy bodies of the Decepticons, hiding the evidence from the unknowing populace of Earth.

The idea isn't entirely sound; it wouldn't be the second first time Megatron's [enjoyed some nice hot water].

From Teletraan I: The Transformers Wiki, a Wikia wiki.