The probability of a mid-sized asteroid (similar to the one that destroyed the Tunguska River area of Russia in 1908) hitting Earth during the 21st century has been estimated at about 30%. Since the Earth is currently more populated than in previous eras, there is a greater risk of large casualties arising from a mid-sized asteroid impact. However, as of the early 2010s, only about a half of one per cent of Tunguska-type NEOs had been located by astronomers using ground-based telescope surveys.