Most natural caves in the UK are either silted up or stained with a natural pigment (peat, etc). Keeping water clear in a tunnel over the course of centuries wi

Most natural caves in the UK are either silted up or stained with a natural pigment (peat, etc). Keeping water clear in a tunnel over the course of centuries without replacing the wall linings regularly would probably be hard if there's anywhere the silt or pigment could come from.