A mystery 300-mile wide formation lurking beneath Australia may be the largest asteroid impact crater on Earth

• Scientists have uncovered traces of what they think could be the world's largest asteroid impact. 
• The structure, hidden in the ground in Australia, is thought to be about 300 miles across.
• The impact might have occurred 450 million years ago, wiping out 85 percent of Earth's species, one said.

The world's largest asteroid impact, 300 miles across, could be hidden in the Australian ground.