Divers of the Dark: The Ojamo Mine Part I

Imagine a temperature around 0°C and a hole in the ice. This is the entrance to the Ojamo mine, the most popular dive site in Finland.

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The Ojamo mine is located 60 km west of the present Finnish capital Helsinki. Every year the site attracts thousands of visiting divers with its beauty. The mine site is part of the town of Lohja, known for its industrial character. The mine itself was started up in the 18th century as an open cast mine, but over time mining was moved deeper and more tunnels were created.

When war broke out between Finland and the Soviet Union in 1939 (Winter War), a prisoner of war camp was set up at the mine site. The prisoners worked in shifts all the time under very difficult conditions. They lived in tents located at the bottom of the pit. As there was not enough appropriate equipment, including clothing for everyone, the prisoners exchanged it between shifts.

Plan of a mine in 3D – Adrian Peret

Today, when the water has filled the tunnels of the Ojamo mine, it looks on the surface like another small forest lake. The shallowest tunnels start at a depth of 28m from the bottom of the open pit. According to the plans, the deepest part of the mine is at a depth of 250m and in the past it was the main mine shaft.

Thanks to the water level, which is level with the ground, the approach to the dive is extremely easy and does not present any difficulties. Divers arriving at the site can park their cars right next to the “lake” and jump straight into the water. Although there are many kilometres of tunnels in Ojamo, the longest railing is only 1.7km long and is located at a depth of 88m.

Characteristic of this place is that the tunnels run practically horizontally, so that depth deviations are very slight. There are many shafts and mining rooms, which make it possible to move from level to level. All this makes diving here much easier to plan than in naturally formed caves. As a result, multi-level plans can be much more accurate.

Stairs leading to the first level, at a depth of 28m – Photo: Divers of the Dark

There are no natural caves in Finland that are accessible to divers. This is because the ground here is solid granite rock, which is too hard and resistant for water to be able to trickle into underground rivers and tunnels. Even the Ojamo limestone mine is surrounded by solid rock, which does not allow carbon dioxide to seep out. From a cave diver’s point of view, this substrate structure has many advantages. The biggest is that all rooms and passages are safe and there is no need for additional support to prevent landslides and collapses.

CDN…

Advertisement by Suunto, made by the Divers of the Dark group at the Ojamo mine site

Source: text prepared with permission and based on materials from the Finnish Cave Divers Group Divers of the Dark