Test are being carried out to ensure homes close to an abandoned mine on Teesside are safe.
The former anhydrite mine, in the West View area of Hartlepool, was closed 75 years ago.
But there are concerns that rising sea water levels within the mine could eventually affect the stability of about 60 properties in the area.
Hartlepool Council, which is conducting the £750,000 survey, has stressed there is, as yet, no evidence of subsidence.
The mine operated between 1923 and 1930 and is 62m deep in places, with a series of connecting tunnels.
But over the years it has become flooded with a mixture of ground water and sea water, which it is feared could lead to the erosion of supporting pillars and roof structure.
One independent survey, carried out in 2000, ruled out any immediate threat to properties.
But a recent rise in planning applications for home extensions has prompted Hartlepool Council to undertake another study.
The authority's director of neighbourhood services, Ian Parker, said: "Erosion of a mine is probably a long-term issue and what we need to do is find out exactly what is happening, so that we are making effective plans for years to come."