The Great Glen Fault in Scotland is the largest tectonic fault in the UK. It stretches across Scotland, includes Loch Ness and is the line of the Caledonian Canal. It's the fault that makes those lochs so long, narrow and deep.
The Great Glen Fault was formed at the time when Europe and North America were rifting apart before the Atlantic Ocean was formed. The fault then became stuck and the land masses above and below which form modern Scotland remained in place. However, the rocks of the fault do slip sometimes producing some of the largest earthquakes felt across the UK.