DOROTHEA QUARRY is one of the finest inland diving sites in the U.K.  Depths vary from 1 metre to approximately 100 metres and the quarry has gained a mixed reputation due to various fatal accidents over the years.  The quarry is a very safe venue providing that divers operate within their training and experienceand plan their dive with regard to the fact that they are diving in fresh water, with it's greatly reduced bouyancy index and temperatures that can reach below 4 degrees Celcius below about 30-40 metres.  Surface temperatures fluctuate areound 6 degrees C.
It is essential that pre-dive planning included emergency actions for free-flow and that divers have ensured that their equipment is properly serviced and IS SUITABLE for cold water.  Regretably, many  modern plastic regulators are heavily prone to free-flow due to their high insulation qualities that allow the internal temperature to fall below the external water terperature. 

Many of the accidents have been caused by free-flow and the author of this article has determined after 31 years of diving  that the prime cause is plastic regulators.  Given that some plastic regulators with heat shunts have proved their worth in the Antarctic (Poseidon etc.) , not all plastic regulators are rogues, however inexperienced divers are advised to take no notice of advertisements and articles showing regulators being tested at depths of  50 metres with multiple divers breathing from one regulator in continental waters.  If they were tested with multiple divers at 4 degrees in Dorothea the test would mean something - otherwise beware.  Many training exercises in cold water dramatically increase the risk of free-flow.

Dorothea is not approved for diving by any diving organisation that the author knows of but is regularly dived by BSAC clubs, SAA clubs and numerous schools at their own risk.  This map is reproduced in an effort to prevent accidents through awareness and good planning.

The photograph is taken from a vantage point to the South East of where divers normally park. It shows vehicles on top of the hill and the pathway down to the concrete slap where divers prepare and kit up to enter the water.

There are two entrances to the site - the first is from Tal Y Sarn and the second is from Nantlle Village. The quarry is situated between the two villages.

A little know fact is that there is a second quarry situated on the other side of the causeway where the pumphouse is visible. S.W section of the plan indicates. (The original Dorothea) This other quarry is more difficult to access and the easierst way to discover it is to have a walk around the site before attempting to dive it. A small road gives close access, but parking is limited to a small turning circle. An island sits in the middle of this other quarry and it is possible to circumnavigate it in one dive.