Set in a sheer-sided bowl of rock, accessible by climbing gear alone, the Red Lake in Croatia has never revealed the secret of its depths.
Now, mountaineering skills and a Saab Seaeye Falcon ROV will offer the first ever insight into the third largest sinkhole lake in the world.
Many attempts to survey the lake have failed, even thwarting an earlier team of underwater specialists. No one knows how deep it is, nor how it is fed, or if it is connected to other water systems.
A ray-finned fish, particular to the lake, sometimes appears in nearby waterways to give the only clue that an underwater connection might exist.
Solving the mystery became urgent when the level of the lake dropped 80 meters, and sent the Croatia Facility of Civil Engineering in search of answers.
They commissioned top Adriatic ROV and diving company, Neptun-Sub to undertake the daring task of sending down the Falcon ROV. Their mission was to discover the depth of the lake, create a 3D profile, and film the underwater surfaces. Lowering the Falcon down a 300 meter vertical cliff face needed the skills of mountaineers.
Their task was only made possible because the Falcon is small enough to be manhandled, yet powerful enough to perform precise work, carrying a wide range of sensors and tools such as high specification cameras and sonar systems as well as manipulators. It is packed with advanced technology usually found in much larger underwater vehicles.
Once the exploration team sent the Falcon diving, it discovered that the Red Lake – taking is name from iron oxide coloring of the rocks within which it sits – is in fact 255 meters deep, four meters of which are below sea level – and it is funnel-shaped.
The filming and 3D survey results, along with data collected by instruments left in the lake, are currently being evaluated by the Croatian Faculty of Civil Engineering, and will be published at a later date.