Yellowstone National Park is warning visitors that elk throughout the park, which are usually pretty safe to view from a distance, have become aggressive and unpredictable as they have entered their seasonal rut. Bull elk can be extremely dangerous during this time. In this condition, bull elk have even been known to charge cars passing near them. If you are on foot and an elk charges, find shelter in your vehicle, behind a tall, study tree or some other sturdy barrier as quickly as possible. Always keep at least 25 yards from any large animal, but at this time, be especially watchful even then for unprovoked aggressive behavior by a bull elk.