<p><strong>A boat makes its way through crude oil on the water's surface on Wednesday, about a week after the&nbsp;<a id="unvm" title="Deepwater Horizon oil rig sank into the Gulf of Mexico" href="http://news.nationalgeographic.com/news/2010/04/100425-energy-oil-spill-sunken-rig-serious/"><em>Deepwater Horizon</em> oil rig sank into the Gulf of Mexico</a>. Even now authorities can only guess at the size of the spill, because the ongoing leak is deep underwater. </strong><br><br>Most <a id="z7vd" title="large spills in history" href="http://www.incidentnews.gov/famous">large oil spills in history</a> stemmed from tanker accidents, and their sizes could be reckoned based on the holding capacity of the wrecked vessels. <br><br>Oil company <a id="ad9t" title="BP" href="http://www.bp.com/">BP</a>, which owns the leaking well, provided the original estimate of a thousand barrels a day, based on underwater cameras that recorded the flow from leaks 5,000 feet (1,524 meters) below water. (See <a href="http://channel.nationalgeographic.com/episode/ultimate-oil-rigs-2487/Overview#tab-Photos/0">oil rig pictures</a>.)<br><br>But the <a id="q9gb" title="U.S. National Oceanic and Atmospheric Administration" href="http://www.noaa.gov/">U.S. National Oceanic and Atmospheric Administration</a>, or NOAA, which has also been monitoring the disaster on the scene and from the air, now says evidence points to the spill being five times worse—about 5,000 barrels a day. BP says it has identified a potentially new leak in the damaged pipes on the sea floor, which it had not seen before.<br><br><em>—Marianne Lavelle</em></p>

Gulf Oil Spill Worsens

A boat makes its way through crude oil on the water's surface on Wednesday, about a week after the Deepwater Horizon oil rig sank into the Gulf of Mexico. Even now authorities can only guess at the size of the spill, because the ongoing leak is deep underwater.

Most large oil spills in history stemmed from tanker accidents, and their sizes could be reckoned based on the holding capacity of the wrecked vessels.

Oil company BP, which owns the leaking well, provided the original estimate of a thousand barrels a day, based on underwater cameras that recorded the flow from leaks 5,000 feet (1,524 meters) below water. (See oil rig pictures.)

But the U.S. National Oceanic and Atmospheric Administration, or NOAA, which has also been monitoring the disaster on the scene and from the air, now says evidence points to the spill being five times worse—about 5,000 barrels a day. BP says it has identified a potentially new leak in the damaged pipes on the sea floor, which it had not seen before.

—Marianne Lavelle

Photograph by Chris Graythen, Getty Images

Gulf Oil Spill Pictures: Aerial Views Show Leak's Size

The spread of oil on the water's surface is a main clue being used to determine the size of the leak from the Gulf of Mexico rig disaster.

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