Archaeologists excavating a cave in the mountains of central Mexico have unearthed evidence that people occupied the area more than 30,000 years ago — suggesting that humans arrived in North America at least 15,000 years earlier than thought.

Deep inside the Rising Star cave system, about 20 miles outside Johannesburg, a group of young scientists led by American paleoanthropologist Lee Berger claim to have found gravesites of small-brained hominids dating back between 236,000 and 335,000 years, much older than the first known human graves from a cave in Israel, which date to about 92,000 years ago.

A team of researchers recently discovered eight previously unknown miles of underground passages in Mammoth Cave, the world's longest known cave system. Led by nonprofit Cave Research Foundation, the discovery expands the total of explored passages within the cave to 420 miles.