And terror-struck in the underworld, Hades lord of the dead
cringed and sprang from his throne and screamed shrill,
fearing the god who rocks the ground above his realm,
giant Poseidon, would burst the earth wide open now
and lay bare to mortal men and immortal gods at last
the houses of the dead – the dank, moldering horrors
that fill the deathless gods themselves with loathing.
So immense the clash as the war of gods erupted.

– Homer

The Iliad, Book 20, lines 73-80. As Zeus allows the gods to return to the battlefield and help whatever side they support, god of the earthquake Poseidon causes quite a stir. He rocks the earth so fiercely that god of the underworld Hades is fearful that he will split the earth and bare his houses of the dead to humans and gods.