We could move this threat from major ASB to minor ASB. How about an early civilization there which makes great advantages in earth moving technologies (explosives in mass production would do)?
They create lots of channels to irrigate the land, so that a population explosion occurs. More and more people level hills and mountains and move the material they can't use for buildings through the channels to lakes and coastal areas to gain more arable land.
In some areas of the coast, the material is swept away by the currents, in others, the water is too deep to cause any significant change for quite some years, so that the effort is given up, but in other areas, it works and more and more material is brought there.
Whoever wants or needs a farm, but didn't get enough land from his parents, buys tons of cheaply available broken rocks and fills some sea area with it. If he's not very successful, someone else continues the job after some time.
Erosion comes along in some parts of the country, as wind speeds pick up, and moves even more materials to the oceans, partly via the irrigation system.
After several hundred (maybe a few thousand) years, all the (extended) Americas, except maybe a few uninhabitable mountain ranges, is a big plain.
Removing the lid from vulcanoes has some bad side effects. At the first instances, the people just gave up on removing material from such sites. But after some time, they managed to use their channel systems to direct the lava to lakes and oceans, to advance their objectives. This also means, the land around the (now constantly active) vulcanoes sinks lower and lower.
A few hundred years later, nearly all of this plain is several dozen meters below water level, with an extensive system of dams at the coast lines. A system of salt water channels and watergates allows easy transport of more material to the coast, while a system of wind driven pumps clears the salt water.
Somewhere, a large earth slide or some other catastrophe makes the dams break, even including the backups (old dams) miles inside the land. The flood wipes away nearly the whole population. The currents and freak waves caused by the floodings also wipe away most of the remaining dams, so that most of the people who fled also die.
In the few places that remain, lack of food reduces the population to nearly none. The few remaining mountains or cold deserts (northern Canada) don't allow the production of the known explosives, so that the few surviving people can't reclaim the land, despite just a few meters to the ground in some areas.
When European explorers a hundred years or so later discover the few remains, they hear of this flooding myth, but don't believe a word. Most evidence has been washed to the sea.
Instead, the area is considered a dangerous shipping ground, because in some places the water is extremely shallow. Only in the 19th century have shipping routes to Asia via the Atlantic/Pacific Ocean been established.
Just today people slowly begin to uncover some faint evidence of cities, channels, tools, and so on across the mentioned area.