I'm trying to figure out what sort of mechanism might...

[töff]

Melted polar caps comes to mind.

So do comets, which can contain a lot of water (ice). You might have one or many comets get disturbed out of stable orbit and crash into your planet.

You might also be more gentle about it, and postulate a big ring of ice particles (such as Saturn's rings) that your planet passes through. Imagine the Asteroid Belt, but smaller and wetter (icier). Again, you'd need an astronomical event to bring your rocky planet into proximity with such a mass of ice particles.

Then there's always artificial hydration. Maybe fish aliens are trying to terraform the planet. Let the alien engineers haul in some water!

[Ascension]

At the extreme edge of plausibility you could have a giant earthquake swallow a whole continent, thereby pushing water levels up. Or you could have some volcano spew out some enormous amount of lava. Neither is all that realistic but it avoids giant craters and fires left over from impacts. My first thought was as Toff says...icy comets.

[guyanonymous]

At the extreme edge of plausibility you could have a giant earthquake swallow a whole continent, thereby pushing water levels up. Or you could have some volcano spew out some enormous amount of lava. Neither is all that realistic but it avoids giant craters and fires left over from impacts. My first thought was as Toff says...icy comets.

That's why I'm wondering about size of particle and where the residual matter ends up....if everything can burn up in the atmosphere, but the water end up within the atmosphere, I don't have to worry about craters - which, really, I don't want to do, as I've got the planet already designed.

I, obviously, should have mentioned that earlier.

Initially the planet will be fairly cold, with most of the residents located close to the ocean using it's energies to generate heat, food, etc.

There are several billion residents of a fairly old civilization.

Then the "incident" happens.

90% of population killed....water levels risen (covering up most cities)...in subsequent years, starvation, disease, nature's turmoil at massive climate change, kills of most of the rest.

1000 years later, scattered small groups survive, though almost all under 5 people (mostly family groups). They are hunters/gatherers/ and exist at an animalistic level in terms of survival of the strongest.

There's the nutshell of my prehistory - but I want to figure out this event before I move forward.

[Nexis]

That's why I'm wondering about size of particle and where the residual matter ends up....if everything can burn up in the atmosphere, but the water end up within the atmosphere, I don't have to worry about craters - which, really, I don't want to do, as I've got the planet already designed.

I, obviously, should have mentioned that earlier.

Initially the planet will be fairly cold, with most of the residents located close to the ocean using it's energies to generate heat, food, etc.

There are several billion residents of a fairly old civilization.

Then the "incident" happens.

90% of population killed....water levels risen (covering up most cities)...in subsequent years, starvation, disease, nature's turmoil at massive climate change, kills of most of the rest.

1000 years later, scattered small groups survive, though almost all under 5 people (mostly family groups). They are hunters/gatherers/ and exist at an animalistic level in terms of survival of the strongest.

There's the nutshell of my prehistory - but I want to figure out this event before I move forward.

Ok how about this.

You say the world is a cold world. This could be the apex of a long growing ice age of the world. Most of the north and South hemispheres are under massive ice sheets, This will bring the sea level down a lot. Our last ice age reduced the sea level well over 300 m. Now the population are gathered mainly along the equatorial belt which has a cold temperate climate at best. This will explain the use of the sea for heat and power. Now you need some thing to happen that is quick but not so sudden, like an impact, that will destroy all things. I propose that the ice is crushing the continental plates together and now one of the mid sea faults fails and splits wide open causing a massive earthquake, tsunamis and volcanic action that cascades along all the major faults spewing forth magma, hot gases and a lot of other nastiness. This quickly heats up the seas and air melting the glaciers very quickly raising the sea level at an alarming rate. As the pressures dissipate the volcanic activity subsides over time but the big melt is unstoppable as the air and sea are warmer and greenhouse gasses are heating up the air. This disaster will take a few years to complete and devastate crops technology and displace and kill lots of people. All the water that was trapped in the ice is now flooding all the lowlands that were once coastline and if the land was low to begin with... Man that was long but fun.

[ottobot]

That's why I'm wondering about size of particle and where the residual matter ends up....if everything can burn up in the atmosphere, but the water end up within the atmosphere, I don't have to worry about craters - which, really, I don't want to do, as I've got the planet already designed.

I, obviously, should have mentioned that earlier.

Initially the planet will be fairly cold, with most of the residents located close to the ocean using it's energies to generate heat, food, etc.

There are several billion residents of a fairly old civilization.

Then the "incident" happens.

90% of population killed....water levels risen (covering up most cities)...in subsequent years, starvation, disease, nature's turmoil at massive climate change, kills of most of the rest.

1000 years later, scattered small groups survive, though almost all under 5 people (mostly family groups). They are hunters/gatherers/ and exist at an animalistic level in terms of survival of the strongest.

There's the nutshell of my prehistory - but I want to figure out this event before I move forward.

well your planet was cold at the time of the event, so it would have lots of its water locked in ice caps. simply have global warming melt the ice caps, raising the water level quite a lot.