Possible Clues:

• Underground place, mostly nasty and hot on the inside
• Site of an underground pool, maybe
• Chamber to expand endlessly when storing too much!
• Cave reached round middle of afternoon
• Cave where empty gear is found by German chap
• Cave reached after crossing river
• Small cave with religious connotation
• Got rot (anag)
• Crossing river, arrived at small cave
• There's one at Knock and Lourdes