“A massive and potentially unprecedented environmental disaster”

GALLERY: The spreading oil slick in the Gulf of Mexico

<p>In this aerial photo taken in the Gulf of Mexico, more than 50 miles southeast of Venice on Louisiana&#8217;s tip, the Deepwater Horizon oil rig is seen burning, April 21, 2010. (AP Photo/Gerald Herbert)</p>

In this aerial photo taken in the Gulf of Mexico, more than 50 miles southeast of Venice on Louisiana’s tip, the Deepwater Horizon oil rig is seen burning, April 21, 2010. (AP Photo/Gerald Herbert)