A churning pool of water has opened up in Yellowstone National Park – just two days after scientists were walking around on the exact patch of ground. The spot where they were standing was replaced by a silt-gray pit the size of a small swimming pool, hissing and thumping with boiling water. The weird part? Nobody saw it happen. The new pool appeared in the Biscuit Basin sometime between June 14 and 16 – shortly after the area was rocked by a sm…