Mud Flood Dungeonry

"With a rumble so low as to be inaudible, growing, throbbing, then fuming into a thundering roar, the earthquake starts..... only it's not like any earthquake in recorded history.

In California the mountains shake like ferns in a breeze; the mighty Pacific rears back and piles up into a mountain of water more than two miles high, then starts its race eastward.

With the force of a thousand armies the wind attacks, ripping, shredding everything in its supersonic bombardment. The unbelievable mountain of Pacific sea-water follows the wind eastward, burying Los Angeles and San Francisco as if they were but grains of sand.

Nothing - but nothing - stops the relentless, overwhelming onslaught of wind and ocean.

Across the continent the thousand mile-per-hour wind wreaks its unholy vengeance, everywhere, mercilessly, unceasingly. Every living thing is ripped into shreds while being blown across the countryside; and the earthquake leaves no place untouched. In many places the earth's molten sub-layer breaks through and spreads a sea of white-hot liquid fire to add to the holocaust.

Within three hours the fantastic wall of water moves across the continent, burying the wind-ravaged land under two miles of seething water coast-to-coast. In a fraction of a day all vestiges of civilization are gone, and the great cities - Los Angeles, San Francisco, Chicago, Dallas, New York - are nothing but legends. Barely a stone is left where millions walked just a few hours before.

A few lucky ones who manage to find shelter from the screaming wind on the lee side of Pike's Peak watch the sea of molten fire break through the quaking valleys below. The raging waters follow, piling higher and higher, steaming over the molten earth-fire, and rising almost to their feet. Only great mountains such as this one can withstand the cataclysmic onslaught...

...New York lies at the bottom of the Atlantic, shattered, melted by earth-fire, and covered by unbelievable amounts of mud. Of San Francisco and Los Angeles, not a trace is left.

Egypt emerges from its Mediterranean inundation new and higher - still the land of the ages. The commonplace of our time becomes the mysterious Baalbek of the new era."

-The Adam and Eve Story by Chan Thomas (partially declassified by the CIA in 2013)

There's a somewhat controversial archeological/historical theory which posits that every few hundred or thousand years human civilization worldwide is annihilated and buried by a great flood. The typically-attributed agent of this destruction is disruption of the Earth's magnetic poles and/or core, both of which are happening right now - nothing to be concerned about I'm sure.

Regardless of this theory's truth in reality, it makes a neat basis for worldbuilding that lends itself to dungeon-delving:

Why would people bother to build these huge underground complexes? Easy - they weren't built underground, they were buried.

Why are there monsters down there? They're surviving fauna of the world before the latest cataclysm, or perhaps creations of the buried civilizations. Maybe there's even a totally-separate chthonic biosphere that periodically gorges itself on the mass death and burial of the surface world.

Why is the treasure and magic in dungeons better than what you've got outside them? The dungeons were buried at the height of their civilization, while you, whether you know it or not, are by comparison a feral child scrabbling about their ruins.

Why haven't all these dungeons been looted already? There's a fresh crop of them made every so often, and only the slow work of wind, water, and geography reveals them from within their muddy shells.

Perhaps in the highest mountains there are monasteries older than any memory, where hidebound orders struggle against entropies both fast and slow to preserve knowledge of past worlds - invaluable advisors in the early stages of the cycle, putting humanity back on its feet, but put aside and disregarded as it explodes into the refreshed and fertile lowlands, distracted with the concerns of the here and now from the apocalypse fermenting far beneath.

Perhaps new stars hang low in the heavens, shining stations set at the apex of their makers' glory. They watch the goings-on of the world, and sing songs of invisible light to mechanical minds sunk deep in the soil, the tunes distorted, out of key. What waits in the bellies of these iron angels? Weapons? Medicines? Long-sleeping kings, waiting for the right sign to descend and conquer?

Perhaps the cycle of global death and rebirth can be broken, the clues read in the darkest depths of the Earth. Was it not silent Nature, but the hand of Man that set the cycle in motion? Can what was done be undone? Could it have even been a mercy, some temperate medium between continuing to exist with the knowledge of some horrid truth, and total extinction?