The first ever reports of the tar pits were from Spanish explorers in 1769, by the Portala Expedition. They reported seeing bones within an asphaltic or tar substance, and even said to have witnessed, some animals becoming trapped or temporarily stuck. Through the 1800's, as civilization began to populate the Southwestern United States, the tar pits were utilized as a commercial commodity. The tar like substance was harvested to use in roofing, and as a sealer against water. The bones discovered within the tar were dismissed as bones from unlucky cattle or horses.