The giant hole spreads across the driveway and front garden of a property on Fontmell Close and is 33ft (10m) deep.
Hertfordshire Fire and Rescue Service said at least 10 people have been evacuated to Batchwood Leisure Centre.
More than 50 properties are without gas and electricity, according to a fire service spokeswoman.
The fire service said it was already aware of a small hole on Wednesday and it was due to be filled in, but it then received a call at 1.30am to say it had “opened up substantially” overnight.
It is the latest in a spate of sinkholes to open up across the UK in the last year.
In August a large hole reported to be 40ft deep appeared on a busy road in Manchester city centre.
Earlier in the year Traigh Golf Course, near Mallaig on Scotland’s west coast, was left with £16,000 of damage after a hole damaged part of the fairway.
In February last year another Hertfordshire garden collapsed as a 20ft deep hole opened up in Croxley Green, near Watford.
In the same month a house in Ripon, North Yorkshire, was left close to collapse after a hole was found underneath the structure.
Peter Hobbs, from the British Geological Survey (BGS), said: ``Sinkholes are caused by dissolution of rock such as chalk. Alternatively, they can be caused by collapse into former mine workings (for flint within the chalk). Changes in the water table may affect the stability of underground cavities.
“Recently, the weather in this part of the country has been relatively dry and water tables may have lowered. The strata at the site consist of about 4m (13ft) of sands, silts and gravels overlying chalk. It isn’t clear at present which of these factors could be the cause. Sinkholes in this chalk formation are not uncommon.”