国际英语新闻：Explosion at U.S. coal mine kills 6, 21 missing

It is reported that the mine has caches of extra oxygen along emergency escape routes. It also has airtight refuge chambers to provide enough air to keep miners alive for four days.

Massey Energy, the fourth largest coal producer in the United States according to revenue, operates 35 underground and 12 surface mines in West Virginia, Kentucky and Virginia, according to the company website.

Mine deaths are relatively not common in the United States. Last year, the number of U.S. miners killed on the job was 34, 18 of them died in coal mines. Notable accidents in recent years include West Virginia's Sago Mine methane explosion, which killed 12 miners in 2006. Six also died in Utah's Crandall Canyon mine cave-in in 2007.