Vapor plumes near sheepfolds in Eryri are from old railway tunnel ventilation shafts.
Actually, the plumes are not natural. Dragons aren’t the cause either. The holes are manmade structures. They’re part of old engineering.
The Ffestiniog Tunnel is under Moel Dymogydd. This mountain is near two towns. The tunnel was very long once, used by trains between towns. Its highest point is deep inside.
This tunnel needed air for steam trains. So, they dug shafts to ventilate it. These shafts went up the mountain.
Steam trains stopped using it a long time ago. But walkers might feel old trains because wind moves through the tunnel’s open ends. This airflow makes breezes. It is warmer inside during winter days, and cooler inside in summer. Even diesel trains add warmth.
Rain falls a lot there. The Ffestiniog Tunnel has water problems. Rockfalls and landslides caused closures; it shut down in 2017 and 2019. The shafts let in moist air, which cools as it rises, causing vapor.
Farmers built sheepfolds back then. They used rocks from digging the shafts. These folds are important culturally, however, the structures are falling apart.
Hidden shafts exist all over Wales. Railway tunnels often have them. Many tunnels date back to the 1800s. About half likely have shafts. Some shafts are easy to see. Others are sealed, but you can find them. The most dangerous are hidden shafts because people forgot about them.
These started during tunnel work. They helped workers dig faster. Workers dug from inside rather than from the ends. Tunnels were almost finished. Many shafts turned into vents. Others were abandoned without records.
Hidden shafts are truly dangerous. One collapsed near Manchester in 1953, making a big hole, twenty feet deep. Two houses collapsed as well, and sadly, five people died at that time.
A study found 86 tunnel shafts in Wales and identified open, blind, and hidden shafts. Some shafts were just suspected. Network Rail started checking onsite. They may need to make changes for extra safety.
They lowered the towers at Ffestiniog. People installed safety railings on top. People used to inspect shafts by rope, checking the structure and taking photos. Now, they use cameras remotely instead, which helps avoid rockfalls.
The shafts are narrow, about five feet wide, and have brick linings up above. The bottoms are rocks. The Conwy Valley Line opened slowly; workers built this tunnel between 1874 and 1879. It took slate from quarries. The line has many passengers today. Nearly 120,000 people use it each year, many of them tourists.