What is coal block allocation scam in which ex-PM Manmohan Singh was given clean chit?

The Supreme Court has given clean-chit to late ex-PM Manmohan Singh and five others in coal block allocation case. The case popularly known as "Coalgate" scam, refers to alleged irregular allocation of Talabira-II coal block in Odisha to Hindalco Industries by UPA Government between 2004 and 2009. Instead of conducting auctions, many coal blocks were allotted through screening committee process.