William B. Sanborn was standing in a marsh in Yellowstone National Park when he saw a hazy patch of blue light sweep toward him at several feet per second. The patch was 150' wide and almost a 1000' long. As it reached him, Sanborn noticed a drop in temperature and the odor of ozone. Keeping low to the ground the cloud flowed around him as well as other obstacles enveloping them in a strange halo of pulsating light. Sanborn could feel his scalp tingle, and felt the snapping of tiny sparks as he brushed his hair with his hand, but obtained no shock from touching any object on the ground or outside his car.