May 31, 2007, was another lovely morning in Yellowstone National Park, with somewhat clear skies and warm temperatures.

Like the Sept. 9, 2006, eruption, nearby Grotto Geyser had ended an marathon during the night, and that created an opportunity for Giant to erupt that day. In this case, the "Hot Period" (pictured above), the sign that it might erupt, came unexpectedly early during that window of opportunity. Mastiff Geyser rose quickly to the height of Giant's cone, far left, then erupted, left.

Below, a photo series showing Giant starting, backlit by the sun.

Far left, Giant and Mastiff erupt together, left, Catfish Geyser joins in after Mastiff Geyser slows down.