Mysteries Underground

The  River Alyn, a major tributary of the Dee, flows over an area of limestone rock near Mold. Limestone can be dissolved by rainwater, which create caves underground! There are three cave systems near and beneath the River Alyn, close to Loggerheads Country Park. 

The Disappearing River
For many months of the year the River Alyn seems to dry up. So where does the water go? Watch the video below to find out!
The dry river bed of the Alyn

Video: Where does the river go?
by Ian Adams, UCET

The Underground World
In the limestone rock near and beneath the River Alyn, the water has eaten the rock away to create a network of caves. Some of these have been explored by caving groups - and there are probably many more waiting to be discovered deep beneath the ground!
Map showing the course of the River Alyn and the two nearby caves
Map overlay: Cris Ebbs and Marc Carney 

Ogof Hesp Alyn (Cave of the Dry Alyn)
This cave is 2 km long, the longest in the area, and was discovered in 1973.

Click the
photo to enter the cave!
Ogof Hen Ffynnonau (Poacher's Cave)
This cave is almost 1 km long, and has some fascinating features, including a gigantic stalagmite base and many fossils in the walls.
Click the photo to enter the cave!

Play the Video
to take a trip down
another nearby cave

The caves formed because the limestone rock in the area can be dissolved by water.  Most cave systems in the world are found in areas of limestone. Limestone is a rock that is formed at the bottom of oceans from the skeletons of small sea creatures.

A map showing where limestone can be found in North Wales An animation showing how a cave can be formed in limestone

The caves near the River Alyn were actually formed in ground that was completely filled with water: the rock was dissolved by the water to create tube-like passages (O-shaped). They are different from caves created by water running through rock as streams or rivers, as shown in the animation above - those caves are more V-shaped.

From the beginning of the twentieth century, water levels in the area dropped because a tunnel was built to drain water from the many mines in the area - this is the Milwr Tunnel, which runs for 10 miles to the sea at Bagillt, and was begun in 1897.
When the water levels fell, the caves near the Alyn drained of water and could then be explored - but after heavy rain they can completely fill with water again!

The Milwr Tunnel, which drains the whole area below ground. Today it still carries 23 million gallons per day to the sea, rising to 36 million gallons in wet weather.

Click HERE to find out much more about the Milwr tunnel.

Powell's Lode Cavern, a gigantic cave discovered while building the Milwr Tunnel - no-one knows how deep it is! 

It is the highest natural underground chamber in Britain.

Limestone Caves
The websites below are just a few of those worth visiting to find out about the spectacular world of limestone caves.

Postojnska Cave  - interactive!
Wikipedia List of Limestone Caves
BBC Video - Planet Earth
Photos - Carlsbad Caverns

Limestone Caves in the UK
Ingleborough Show Caves
Poole's Cavern
Wookey Hole
Cheddar Caves

Cave Formations
Interactive Cave features map
The Virtual Cave

And finally: a great claymation video from an Australian boy about cave formation

UCET (United Cavers Exploration Team)
Subterranea Brittanica
Caves of North Wales

Many thanks to Ian Adams and UCET for permission to use their photos and videos, and for lots of fascinating information.