The 66ft (20m) diameter hole spread across a front garden and driveway on Fontmell Close and is 33ft (10m) deep.
Hertfordshire County Council said five homes were evacuated and 20 people were taken to a centre set up at the nearby Batchwood Sports Centre.
Residents are said to have heard a crash before the huge crater appeared.
Hole opened up 'substantially overnight'
A spokesman for the fire service said it was previously aware of a small hole that was due to be filled in.
Firefighters received a call at 01:30 BST to say the hole had "opened up substantially" overnight.
A Hertfordshire County Council spokesman said: "We were informed about a hole in the footpath last week. It was inspected and barriers put around it on Monday.
"Work was scheduled to fill it in this morning. Following our inspection, there was no reason to suspect that the hole would collapse."
Heavily pregnant Gemma Bagshaw, 32, is expecting to give birth any day now and she was woken by the sound of the noise.
She and her husband Ben left their house clutching her hospital notes and bag that was packed in readiness for a trip to hospital.
"We were asleep when we were awoken by this awful rumbling sound like an earthquake," she said.
"Ben jumped out of bed and got to the window, then I heard him say 'The road has gone.
"If this doesn't bring on my labour then nothing will."
Steve Lynch, who lives nearby, told BBC journalist Tony Fisher a postman had stepped in the smaller hole.
He said: "My wife saw the local postman pulling himself pretty much out of the hole and dusting himself down... he was rolling around on the street looking quite badly injured."
Highways engineers are currently assessing the scene before the crater is filled in.
Fifty-two homes in the area were without gas, electricity and water, including residents on nearby Bridle Close.
"The return to normal is likely to take weeks and all agencies in Hertfordshire are working together to support families affected," the council spokesman said.
He added no-one had been injured and no homes were damaged.
A massive sinkhole has opened up on a street in St Albans, forcing the evacuation of several families from their homes in the night.