So far in this series we've been up mountains, to the beach and have seen sunsets. Nothing for it - we need to go underground! Here's something you don't see everyday, an underground waterfall. This is the wonderfully named Smoo Cave at Durness, which is a unique geological feature in the UK as it was formed by both the action of the sea and by freshwater. The large outer cave was formed by the actions of the sea, and is in fact the largest sea-cave entrance in the UK. The inner chambers of the cave, including the waterfall chamber shown here were created by freshwater eroding permeable rocks. The waterfall is the Allt Smoo falling into the cave through a hole in the cave roof. On my first visit to the cave the Allt Smoo was in spate due to heavy rain, and the spray that it kicked up was incredible - it soaked in seconds so there was no chance of photography. It is an amazing place, well worth a visit if you are ever up that way!

Best Viewed On Black here The amazing cave waterfalls at Smoo, this is an amazing place, I came in the evening and there wasn't a soul around, had the place to myself, one of the most attractive places I've ever been to in Scotland. Smoo Cave is a large combined sea cave and freshwater cave lying 2 miles from the village of Durness on the Northern tip of mainland Scotland. 30 second exposure @ F11