A unique natural wonder nestled in the heart of Transylvania has officially secured a global title, as Bear Lake (known locally as Lacul Ursu or Medve-tó) has been certified as the World’s Largest Heliotherm Lake.
The lake, renowned for its therapeutic, sun-warmed salt water and its distinctive shape, has an area covering approximately 40,235 square meters (about 10 acres), making it the most extensive natural body of water to exhibit the rare heliothermic phenomenon.
The Bear Lake’s world record status is entirely dependent on its unique hydro-geological properties, which allow it to absorb and store solar energy .
Bear Lake was formed following the collapse of a salt mine in 1875, leading to the creation of a depression that filled with rain and stream water.
The two streams flowing into the lake create a shallow, less-dense layer of freshwater on the surface. This freshwater layer acts like a magnifying glass, allowing the sun’s rays to penetrate and warm the highly saline, denser water below. Crucially, the freshwater acts as an insulating layer, preventing the heated saltwater from rising and cooling via contact with the air.
This natural greenhouse effect can maintain high temperatures in the lower layers, sometimes reaching up to 35°C (95°F) at a depth of a few meters.
Located in the popular spa town of Sovata, the lake’s water and mineral-rich mud have been used for over a century for their alleged healing properties, treating various conditions including rheumatism and gynecological disorders. This has earned it the local nickname, the “Dead Sea of Transylvania.”
The lake is also famous for two other remarkable facts: it is the only salt lake in the world surrounded by rich vegetation due to a layer of volcanic soil deposited over the salt mountain, and it is the only natural lake in the world whose exact date of formation is known (May 27, 1875).
To preserve the delicate thermal structure necessary for the heliothermic effect, strict rules are in place, including restrictions on bathing hours to allow the lake’s freshwater layers to rest and reform.
The recognition solidifies Bear Lake’s standing as a marvel of natural science and a jewel of Romania’s protected natural monuments.
