Part of Heathrow's £4bn Terminal Five site had to be evacuated after the discovery of an unexploded anti-aircraft shell.
Terminal 5 is the size of Hyde Park
Work on the project ground to a halt for a few hours, after builders dug up the shell, dating back to World War II.
Thousands of workers were moved to safety and a 200m exclusion zone was put in place while explosives experts carried out a controlled blast.
There were no injuries, and Heathrow flights were unaffected.
The new terminal, being built to the west of Heathrow, covers an area the size of Hyde Park, has stuck to budget and is slightly ahead of schedule, say developers Laing O'Rourke.
The departure lounge will partially open in 2008 - although it will be 2011 before the project is entirely complete.
The new terminal will serve an extra 30m passengers a year - critics have said it will mean more air travel and pollution.