A "possible void" has been found under a road where a sinkhole opened up, which could be three times larger than the existing hole, a council said.
The 33ft (10m) deep hole appeared in the former claypit site of Fontmell Close in St Albans on 1 October.
The county council said a survey showed a "significant anomaly" which was likely to be underneath homes.
It said it was talking to residents about carrying out "intrusive" surveys as soon as possible.
A survey of the road and footpath in Fontmell Close and Bridle Close by Geotechnology Ltd involved working out the precise measurements of gravity under the road.
It showed the collapse, which has since been filled with 48 lorry-loads of foamed concrete, happened within the boundary of a clay pit excavated in the 19th Century and backfilled with waste from the area.
However, experts said the "most likely" explanation for the collapse was chalk excavation in the bedrock beneath the clay.
The council said its investigation appeared to show "a significant amount of missing mass adjacent to the site of the collapse".
Rob Smith, the deputy director of the environment at Hertfordshire County Council, said: "We don't know where the centre of the anomaly is but it tells us there is something there that requires further investigation.
"The best interpretation of the data is that there is a void of about 20m depth, which is likely to be an old chalk mine, but this is not definitive."
He said it was likely that was underneath homes but the council needed to carry out a "more intrusive investigation" to "determine the full extent of the problem".
The council said it could not give an exact timescale but it would "be at least six months" before the "anomaly" could be investigated and fixed.
Mr Smith said the authority had met with residents to discuss the way forward but if a cavity is found to extend under houses, householders must speak to their insurers about remedial work as the council is only responsible for the highways.
"We appreciate this is upsetting news and are continuing to work together to determine the next steps, he said.
"It's a dreadful situation for the residents, I would love to have a magic wand and make it go away but we are being as informative as we can."
Most residents in the close have returned to their homes but it is not known when the few people in the immediate area of the hole can return.
Mr Smith said "a number of anomalies" had been identified in the area which also needed investigation but they were not as urgent.
"If residents are worried they need to undertake their own investigation," he said.
"But I am happy that they have all the information from us so they can make their own decisions."
Rosemary Broom who lives adjacent to the hole said she is concerned about the situation.
"We don't really know where we're going with it at the minute, it's very worrying," she said.
Neighbour, Ben Bagshaw, said: "We're hoping the insurance companies will help us out, but so far there's been no promise that's going to happen."