Work to cap 12m wide sink hole near Rhes-y-Cae school
Work is under way to cap a 12m (39ft) wide sink hole, which opened on land next to a primary school in Flintshire.
Mineshaft specialists started working near Ysgol Rhes-y-Cae on Halkyn Mountain following months of monitoring.
The hole appeared in September, a year after a 7m (23ft) wide hole opened up on the same site.
A decision to remove fencing around the area will not be made until after a further six months of monitoring.
Last week, a small hole opened on grounds at nearby Holywell Golf Club.
"Fortunately, mineshaft collapses on the scale of the one in Rhes-y-Cae only occur rarely, but smaller incidents do happen from time and time," said Edwin Christmas from Grosvenor Estate which is the landowner of the area of the sink hole.
"Indeed, another small hole opened last week on the grounds of Holywell Golf Club.
"These types of incidents demonstrate the need for us to carefully manage the area to minimise any potential risks to the safety and interests of land users."
Since the latest incident at Rhes-y-Cae, the landowner said it has been working with specialist consultants, local police and community, including the school and residents, to ensure public safety.
Residents say the area is known to contain disused mine shafts and other shafts have collapsed and opened up on Halkyn Common in the past.
Mr Christmas said: "Whilst our experts' guidance is that there are no substantial dangers for people who use the land at Halkyn Mountain, the safety of local people and visitors is of paramount importance to us."
The estate said previously that following expert advice and extensive surveying the first hole was covered with a purpose-made concrete cap in August 2010, constructed to industry best practice standards as adopted by the coal authority.