We’re still in Yellowstone Park, looking at the spring water sources. If I’m interpreting this one correctly, there are at least two different subterranean channels feeding this pool, apparently passing through different types of minerals/metals. The one centered in this image, I’m almost certain, is showing a high iron content, so yes, that’s rust. But as you already know if you’ve followed enough posts here, trusting me is a stupid thing to do. Some tourist probably just dumped their pumpkin spice latté into the pool, and it ate that hole into the stone.