The first two miles of the divisive HS2 project have been completed, according to the firm building the high-speed railway.
HS2 Ltd said Florence, the first giant machine launched, has excavated 1.3 miles of tunnel under the Chiltern Hills, while the second, Cecilia, has made 0.9 miles of progress.
The pair of tunnel-boring machines (TBMs) are expected to complete twin 10-mile tunnels in around three years.
The 170-metre-long machines work continuously, operating as a self-contained factory cutting through a mixture of chalk and flint.
Florence was launched at a site in Buckinghamshire near the M25 motorway in May.
She was named by local schoolchildren after Florence Nightingale, the founder of modern nursing who spent many years in the county.
Cecilia, named after Buckinghamshire-born astronomer and astrophysicist Cecilia Payne-Gaposchkin, was launched in July.
A total of 10 TBMs will be deployed between London and the West Midlands for Phase 1 of the high-speed railway.
Phase 1 of HS2 was due to open in 2026, but in an update to Parliament in 2019, transport secretary Grant Shapps said the opening date would be pushed back to between 2028 and 2031.
Last month the government said that dealing with anti-HS2 protests has cost the high-speed rail project up to £80 million.
Environmental activists caused disruption at several sites on Phase One of the line between London and Birmingham.
An expensive month-long operation was required to remove people from a network of tunnels in London’s Euston Square Gardens earlier this year.
Some £15.3 billion (in actual prices) has already been spent on the project, including through land and property acquisition.
The government-commissioned Oakervee Review warned in 2018 that the final bill for HS2 could reach £106 billion (at 2019 prices).
Despite it running tens of billions of pounds over its initial budget and several years behind schedule, Boris Johnson gave HS2 the green light in February 2020.