In the video above Paul Hsieh, a research hydrologist with a specialty in underground water reserves, discusses his role in helping to contain the worst spill in U.S. history.

It’s an incredible story. In July 2010, a 75-ton containment cap was placed on the Deepwater Horizon well to stop the flow of oil, which had been gushing into the Gulf of Mexico for 86 days.

Problem was, no one knew whether the cap would hold. Hsieh used a modified version of his reservoir modeling software to do complex calculations of the well’s pressure curve, which he determined by examining a photo sent from a colleague’s cellphone. Hsieh worked through the night from his office in California. After hours of analysis, Hsieh concluded the cap would hold and was not leaking beneath the Gulf surface. The cap remained in place and indeed never spilled another drop of oil.

“Paul’s model provided the confidence for the government team to keep the cap and stack closed,” said Rear Adm. Kevin Cook, director of prevention policy, U.S. Coast Guard. “It was a real game changer.”

Read the full story in this citation when he was named 2011 Federal Employee of the Year. Hsieh earned his undergraduate degree in civil engineering from Princeton in 1977.