HS2’s two TBMs now three-quarters of the way under the Chilterns

The two tunnel boring machines that set off from just inside the M25 motorway are now three-quarters of their way under the Chilterns as they head north out of London.
At the tunnel entrance just inside the M25 motorway
The two TBMs are digging two tunnels for the HS2 railway and have now reached the Little Missenden ventilation shaft, more than three-quarters of the way through their 10-mile drive under the Chilterns.
The two machines set off at staggered times in early 2021 and have now caught