Smoo Cave, located in Durness, Scotland, is a cool combo of sea and freshwater caves. The name probably comes from the Norse word for "hole" or "hiding place." The cave formed in dolomites and has a mix of sea-carved chambers and rainwater-dissolved passages. It's got a big entrance chamber, a waterfall chamber, and a freshwater passage leading to a sump chamber. Archaeologists found Neolithic, Norse, and Iron Age artifacts in there, suggesting it was a happening spot back in the day. Nowadays, it's a popular tourist spot with around 40,000 visitors each year, and you can even take a boat tour to explore the inner chamber.