tunnel in an underground rock formation illuminated with artificial purple-pink light

abstract ancient cave cavern caving closeup deep discovery earth environment erosion europe exploration explore flowstone formation geologic geology groove grotto hole interior limestone massif mountain mystery natural nature old park pattern perspective ravine reserve rock rocky rough slope spectacular speleology stalactite stone surface tourism travel underground underworld wall wilderness tunnel illuminated artificial purplepink light cracked uneven walls water seepage neptune caves couvin wallonia belgium