Since the continental plates have been drifting for a long time, continents have merged into more supercontinents which later broke apart again more than once.

Pangaea is the most recent, which began breaking up in the days of the dinosaurs. It is also one for which we have one of the most accurate maps. First link below.

Rodinia is the second youngest, also the second link below.

Although continents can rise and fall slightly due (say) to the weight of glaciers on the continental plates, or may seem to shrink or expand as sea level changes due to glaciation, no land sank or is missing. Basically continents float around on continental plates - picture bumper cars with continents on the hoods.

The Gulf of Mexico is believed to have been formed due to the North American continental plate cracking.
Source(s):
http://en.wikipedia.org/wiki/File:Pangaea_continents.png
http://www.palaeos.com/Earth/Geography/Rodinia.htm
http://en.wikipedia.org/wiki/Gulf_of_Mexico

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