(CNN)Black gooey methane bubbles pop on the surface of the Lake Pit outside the La Brea Museum in Los Angeles. It's the only warning of the sticky, heavy asphalt on the bottom of the pit. The asphalt seeps have been there for thousands of years, stemming from a nearby underground large petroleum reservoir called Salt Lake Oil Field.
Preserving the unique history of the La Brea Tar Pits
Thousands of years ago, the liquid asphalt trapped unsuspecting mammoths, horses, giant ground sloths, camels and bison that roamed the area, thirsty for a drink. Only 1.5 inches of the asphalt was required to trap a creature as massive as a mammoth or bison.
Predatory carnivores saw the stuck animals as the perfect way to get a meal. Dire wolves, saber-toothed cats and lions would creep up on their trapped prey but they too would become stuck in the liquid asphalt.
It was a slow death by starvation. They decayed on the surface and became buried by the asphalt and sediment over time.
But the unique nature of the La Brea Tar Pits is that they preserved an entire ecosystem between 10,000 to 50,000 years ago, containing massive mammoth tusks and giant sloth bones alongside acorns and microscopic plant and insect fossils. More than 100 species of birds and a number of other species were first described after being found at La Brea.