It is a gargantuan 164 ft (50m) long, 44ft (13.4m) wide and 54ft (16.6m) high at its largest point.
The team of 160 workers, who are lowered into the site in a cage-lift, have worked 24 hours a day, seven days a week on the tunnel, which should be finished in the next few months.
The cavern will mark four years since construction started on one of Europe’s largest infrastructure projects.
It is due to be finished in 2018."
