
Dorothea is one of the very deep diving sites and is within 10 minutes (by the coast guard ASR helicopter) of Bangor hospital who have a barometric chamber for those unplanned emergency ascents.
Not my cup of tea - I get nervous diving more than two meters to scour the sea bed for "stuff" when snorkelling but the videos are impressive! There they dive down to around 106m if they feel the need! This video is I think 60m level where a lot of the quarry infrastructure is still evident.
Moxi