How do people make man made islands?

Here's a brief overview:
http://ask.yahoo.com/20050729.html

And more to read about them here (http://www.google.com/search?hl=en&lr=&q=%22man+made+islands%22+AND+make+OR+made&btnG=Search).

One way is by adding a lot of garbage. That doesn't create an entire island, but it extends the beach front on many properties.