One popular site in the UK is Dorothea quarry. It is a huge quarry that has flooded since it went into disuse years ago. There have been, to date, 25 deaths in it as it contains a number of hazards to catch the unwary such as 100m depth, cold water, machinery, wires, buildings etc (the quarry underwater remains as if it were a working site with all the machines etc that would have been there - it has never been stripped to make it safer).
There is stuff to see nearer the surface but for technical divers with the resources to dive the depths (such as rebreather/stages etc) it is a veritable playpen.
There is stuff to see nearer the surface but for technical divers with the resources to dive the depths (such as rebreather/stages etc) it is a veritable playpen.