Archaeologists exploring the world's biggest flooded cave in Mexico have discovered 9,000-year-old remains from Mayan civilisation including burnt human bones, ceramics, and wall etchings. The caves, connected by 347 km of tunnels, revealed around 200 archaeological sites with remains of an extinct elephant-like animal, sloths and bears from the last Ice Age, 2.6 million years to 11,700 years ago.