Two sinkholes believed to be caused by the boring of tunnels for HS2 have opened up in Buckinghamshire.
The holes were discovered on land where the Chiltern Tunnel is being constructed on the outskirts of South Heath near Great Missenden, roughly 30 miles from central London.
HS2 Ltd said "safety was its top priority" and both sinkholes had been fenced off amid ongoing tunnelling operations, according to the Local Democracy Reporting Service.
The sinkholes may have been caused by disturbance from tunnelling and recent rainfall that exposed existing weak spots, HS2 Ltd has explained.
A spokesman said: "We have always recognised that the geology of this part of the route is particularly challenging - with poorer quality, less stable chalk susceptible to ground movement.
"Temporary fencing has been installed and extra monitoring put in place."
The smaller of the two holes was found next to Frith Hill during routine monitoring on February 7 before a larger sinkhole was found in the same field five days later.
The discoveries mean four craters have been caused by HS2 in the county in nine months.
A five-metre deep sinkhole, which measured around six metres in diameter, opened up by Shadeloes Lake near Little Missenden in May.
Another sinkhole was then found in Hyde Heath in November.
Both were filled in and were found to have no significant impact on water quality.
The 10-mile Chiltern Tunnel is the longest on the HS2 project, which aims to improve links between London and Birmingham.
An Environment Agency spokesman said: “We are investigating sinkholes on private land at Frith Hill in Buckinghamshire to understand whether they have resulted in any pollution to the local environment.
“Our specialist officers work closely with HS2 Ltd and contractors to ensure all relevant legislation is followed. We also continue to talk to local people, the water industry and local authorities to ensure the environment is protected throughout the HS2 project.
“Anyone with new concerns about this incident can call our 24-hour incident hotline: 0800 807060.”
Buckinghamshire Council has been approached for comment.
