AUX -ingThe auxiliary verb be is followed by the '-ing' form of another verb. The auxiliary and the other verb together form the verb group.
This pattern is used to form continuous tenses.
Darkness was coming, a pink glow above the rooftops.
He was being questioned at a police station in London.
Everybody is complaining about the recession.
An air and sea rescue operation has been going on all day for the crew of a fishing trawler which sank in the English Channel.