(p. 4) The coastlines might seem like permanent features of geography. But over the past few million years, massive ice sheets expanded and receded, and seas rose and fell by hundreds of feet. Then, around 12,000 years ago, the most recent of many glacial ages ended, and seas eventually rose by 400 feet.
This is roughly where we are today.
For the full commentary, see:
PETER BRANNEN. “OPINION; Lessons From Underwater Miami.” The New York Times (Sun., APRIL 24, 2016): 4.
(Note: the online version of the commentary has the date APRIL 23, 2016.)