We still don't know exactly how much oil has been spilled into the Gulf of Mexico since the Deepwater Horizon rig exploded on April 20, because there is no way to directly measure the flow. The oil is leaking out of at least two different points along the crumpled drill pipe that used to run between the wellhead and the rig, and estimating flow based on the size of the slick on the sea surface is an inexact science. But some independent experts believe that the official estimate of 210,000 gallons a day could be on the low end. BP itself admitted in a recent congressional briefing that the well could potentially spill as much as 2.5 million gallons of oil each day, more than 10 times the current estimate. If the official estimate is accurate, then the spill's magnitude will surpass that of Exxon Valdez by mid-June, but that milestone may already be past if the flow is actually higher. We may never know for sure, but in the meanwhile, here is a cool widget from PBS that shows how much oil has leaked into the gulf so far based on the leak rate you select.