Sinkholes: what are they?

Sinkholes have been making headlines in the last few weeks. Most recently in North Yorkshire, where a large hole opened up in Ripon, causing a building to partially collapse.

So what are sinkholes?

Well, as the name suggests, they are holes that form under the surface into which the land above literally sinks.

Erosion to rock, subterranean caves and even man-made mines can create the voids that lead to such collapses. And where there are buildings on the land above they can prove to be very dangerous, even fatal.

Sinkholes are hard to predict because the land on the surface can appear normal. The soil above the hole can act as a bridge, holding in place when the rock below has gone.

But, despite appearing to be terra firma, the ground is unstable and can give way without warning. And when they do they take whatever is above with them.

Cars and even whole buildings have been swallowed up, leaving little more than a gapping hole in their wake.

The situation has been made worse by the weather. Heavy rain and surface flooding can cause an increase in erosion underground as well as weaken the land above, factors which can trigger the collapse of sinkholes.

Vanessa Banks, from the British Geological Survey, said: "The ground is saturated at the moment, in certain parts of the country, and where it's saturated, it will remain saturated for some weeks, if not months.

"And gradually the situation will improve, but in the short term I suspect there will be more of these incidents being reported."

In Ripon the local geology is being blamed. The area sits on a huge deposit of gypsum, which is a very soluble rock. This means it is more easily eroded away.

The British Geological Survey says that the area is the most susceptible area in the UK for sinkholes.

Older buildings, like the one damaged in Ripon, are more at risk because they were built at a time when people didn’t think too much about what was beneath them.

Modern buildings are much safer as checks are done on the land on which they are built. They are also designed to be more resistant to subsidence.

Although sinkholes are hitting the headlines at the moment, they are by no way a new phenomenon. The gypsum deposits that make Ripon so susceptible are millions of years old.

Ms Banks said she fears that increased knowledge about sinkholes is worrying people.

"I fear that people are becoming worried because we're increasing their awareness of the occurrence of these things.

"These cavities have existed for as long as we have been developing. They pre-date humans," she said.