Balloon has gone up
This expression dates from the First World War and is now figurative for major trouble ahead. When the troops in the frontlines saw the balloons of the artillery observers go up in order to get a better view of opposing enemy lines, they knew that trouble was brewing. Increased balloon activity invariably signalled a fresh offensive. For the frontline soldiers, the balloons going up were a dreaded and ominous sign. See also All hell let loose and Shit hits the fan.