Emergency services were called to Cedarwood Drive, St Albans at about 05:50 GMT after the hole was spotted by a member of the public.
A spokeswoman for St Albans City Council said people in the Cedar Court flats had been moved as a precaution.
Residents said they expected to be out for at least two weeks while
Structural engineers and crews from the utility services are at the scene.
The previous massive sink hole in 2015 caused month's on disruption caused due to old mines. in Fontmell Close in the city - about 2.5 miles (4km) from Cedarwood Drive. It cost about £600,000 for the local councils to fix.
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