Official websites use .gov
A .gov website belongs to an official government organization in the United States.
Secure .gov websites use HTTPS
A lock () or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.
Cross section of an island’s freshwater lens (blue), showing that when seawater (green) floods it, 15 months or longer may pass before the heavier salt water settles to the bottom and eventually mixes out of the system to make it drinkable.
Public Domain.
