I was on a hike and read a plaque that said this area used to be completely covered by water. My question is, where did all that water go? Just absorbed into the ground? Evaporated?
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It depends.
If water evaporates and falls as rain, it just runs into a river and back into the sea. But if it falls as snow and the snow doesn’t melt, that lowers sea level. The Antarctic ice sheet formed that way about 40 million years ago and dropped sea level about 100 metres.
Another thing ice does is pushes the crust down, and when the ice melts the land slowly rebounds upwards. Much of Finland was covered in ice during the last glacial period, then flooded after the ice sheets melted, but is now dry land.
Over longer time periods, plate tectonics affects the depth of the oceans. Young seafloor is higher – as it gets older it cools and sinks down into the mantle. If the oceans are less deep then the water ends up flooding the continents. This process is part of what made the Western Interior Seaway across much of what is now the USA.
Estimates of past sea level have it about 400 metres above the present day at the highest. To get marine rocks *really* high up required plate tectonics to drive land upwards. When continents collide the rocks crumple forming mountains and plateaus. There can also be gentler upward and downward bending further away from the main collision or through other processes. This helped create the western seaway I mentioned – the sea rose up but the land went down too.
As far as I know there’s no significant change from water entering or leaving the deep earth over the past few hundred million years.
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