Scientists are issuing dire warnings about the scenario of the top kill’s failure. Professor Tad Patzek of the Department of Petroleum and Geosystems Engineering at the University of Texas-Austin told The Hill newspaper that an unchecked leak “would be an environmental disaster of a caliber heretofore unseen by humanity.” President Obama meanwhile has announced he’ll return to the Gulf Coast on Friday, his second visit to the region since the spill began.

President Obama: “A lot of damage has been done already — livelihoods destroyed, landscapes scarred, wildlife affected. Lives have been lost. Our thoughts and prayers are very much with the people along the Gulf Coast. And then let me reiterate: we will not rest until this well is shut, the environment is repaired, and the cleanup is complete. And I look forward to returning there on Friday to review the efforts currently underway and lend my support to the region.”

In other oil spill news, the Coast Guard has ordered all 125 commercial ships helping with the cleanup effort to return to land after several people fell ill.