If you live in the United States and you want to see or even interact with elephants, your best bet will be a zoo or perhaps The Elephant Sanctuary in Tennessee, possibly the closest habitat in the country that resembles a natural elephant environment.

While there are no wild elephants in North America now, gomphotheres — about the size of modern elephants and their ancestors — roamed the land some 13,000 years ago, which is like yesterday in geological time. At the time, many large mammal species, including lions, became extinct in the region in part due to a warming climate, but mainly because of massive over-hunting by the earliest settlers.

At least this is a popular theory. There is no definitive proof that humans were the reason for the rapid extinction of some animals from North America, but skeletons of large mammals have been unearthed in the United States containing, for example, spear points, indicating a connection, according to some geoscientists.

Another possible cause is climate change. These big animals lived at the end of the Ice Age. The climate was rapidly changing and temperatures were rising. Their natural habitat was simply changing faster than they could adapt and eventually the animals died off. (These are other possible disaster scenarios caused by climate change.)

A third suspect is disease. Some scientists, unconvinced by the warming climate or over-hunting theories, argue that microorganisms carried by dogs, rats, bugs, and birds — which came along with the first human settlers in North America — must have killed the animals. (In a way, it’s still people’s fault then.)

In 2007, a fourth theory that involved an extraterrestrial object entered the mix. It has been debated over the last decade or so that a comet or meteorite killed the mega-beasts that once roamed North America. “Spherules,” or small spheres of rock, found in Pennsylvania and traces of platinum in the Greenland ice cap are the basis for this hypothesis.

Fast forward a few millennia and elephants have been brought back to the continent, but they are held in captivity. The first elephant in the United States came from India in 1796. Shortly after that it was purchased so it could be part of a circus. Today, the large mammals, which are known for, among other things, for their incredible memories, live mostly in zoos.

Over the last couple of decades ecologists and conservation biologists have been talking about reintroducing elephants, as well as lions, camels, and other extinct mammals into the wild in the United States. The idea is for a huge ecological park to be developed where endangered animals from Africa would be able to live freely. And here are the most threatened animals in America.