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A second sinkhole has appeared in an up-market neighbourhood south of Newcastle, prompting fears more land could collapse into an old mine shaft that appears to have caused the erosion.
The first sinkhole, measuring up to 20m wide and 10m deep, developed on Tuesday night, next to a three-storey home on Lambton Parade in plush Swansea Heads, near Lake Macquarie.
A husband and wife returned to their seaside property about two hours after the hole developed next to their front deck, swallowing tonnes of dirt and debris - but luckily leaving the house, built in 1997, intact.
A second, smaller sinkhole developed on Wednesday morning in the front garden of a property two doors down. It measured about two metres across.
The area underneath the neighbourhood was once part of the Swansea Pit, a coal mine abandoned in the 1950s.
The Mine Subsidence Board is now leading an investigation into the sinkholes, including checks to ensure the stability and structural integrity of surrounding homes.
"It appears what has happened is that an old mine shaft, a furnace shaft, has caused the collapse," Inspector Sam Crisafulli told the Newcastle Herald.
Sinkholes can occur because of land collapsing into mined areas.
But they can also occur naturally, with acidic rainwater seeping through surface dirt and eroding bedrock.
Large sinkholes can swallow whole houses and their occupants.