This video provides an introduction to the Wilson cycle, a theory that describes the lifecycle of the ocean basin:
As you can see, it is very common for oceans to form and then disappear. Continental rifting plays a key role in the formation of an ocean. But how did the oceans form in the first place? Watch the following video for some current theories:
The low lying areas mentioned in the video are the ocean floors. Oceanic crust is basalt and continental crust is composed granite. Basalt is heavier so when they both rest on the top of the mantle, oceanic crust sinks lower forming a basin for the water to drain into.