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Comparison of (a) 1904 Historical map with (b) 1988 USGS map. Colloidal Pool is a large, labeled pool roughly located on a straight line between Hurricane vent and Whirligig Geyser on the 1988 map (b); this same transect on the 1904 map (a) shows no feature at that location (white circle). Note: maps are not at the same scale or orientation. Panels c and d show photos of Colloidal Pool’s variations in water appearance from the Yellowstone Flickr page. (c) Teal-blue water without foam (photo taken August 27, 2009, by Greg L. Jones). (d) Foamy conditions with opaque blue-brown water (photo taken August 4, 2019 by Joe Shlabotnik).