This is not a rabbit hole or a tunnel leading to secret treasure, as Steven Kempster’s children had hoped.
But it is a six-foot deep hole that had previously been undiscovered in the communal gardens at flats in Comet Close, Leavesden.
Mr Kempster’s daughter Ellie was playing in the garden on Good Friday when he noticed the ground she was jumping on was moving beneath her.
The 37-year-old ushered Ellie out of the way and bounced on the patch himself before the ground gave way, leaving his foot dangling through a gap above the hole.
Mr Kempster said: "It’s the same size as a house so I’m very relieved and feel fortunate that none of the children fell through it.
"I got the kids away and I called a neighbour out to cordon it off but my first reaction was ‘that could have been my little girl’.
"I was more scared for them - had they fallen down there, not only would we need the fire brigade to get them out as it is so deep but the children could have been seriously injured.
"It’s quite frightening thinking about what could have happened and not knowing how long it has been there. Everyone is shocked."
It is unclear what the hole is or how long it has been under the gardens at the block of flats. An investigation is under way.
Mr Kempster said flat owners and tenants were now concerned about whether any other holes would be discovered in the area.
But he added: "The kids love it. Ellie thinks we found a giant rabbit hole and my son asked if it was a treasure tunnel from The Goonies."