The "Green Flash" is an atmospheric phenomenon which occasionally occurs at sunset in Key West - when the last bit of sun disappears below the horizon and a brief green glow is observable. This is usually explained by the refraction of light at that angle appearing green and is only visible during specific atmospheric conditions.
Or, you could believe something else causes it.
XKCD, the online comic dedicated to uber-techie humor, posted their own humorous explanation.
(Be sure to hover over the image to see XKCD's embedded joke text)