We know that the walls are limestone, a porous rock saturated with rainwater.

Oil exists in drops between and inside porous rocks.

The porous rock left when this cooled was brittle enough for chunks to be broken off by hand.

If natural, the sound was probably caused by rising temperatures and the evaporation of dew inside the porous rock.

The walls were porous rock here, and came out in chunks.

The process will be particularly useful in areas of porous rock, where silage pollutants can travel many miles before entering the river system.

If we hit porous rock, we get lots of water.

The elevations in the area are small with mostly porous rock.

They were built along the spring line, where the underlying aquifers met less porous rocks.