The lake never freezes, even during severe Czech winters, due to the chemical composition of the water.
It is one of the oldest known artificial pit lakes in Europe, created after the alum mining operations ceased in the late 1800s.
The water contains high levels of iron and sulfate minerals, preventing any algae or microorganisms from surviving.
The mining site was originally used to produce alum for the textile industry and later for iron sulfate production.
The steep, multi-colored clay banks surrounding the water are a result of oxidation processes from the remaining shale minerals.
Hromnice Lake, known locally as the Red Lake (Červené jezírko), is an acidic pit lake formed in an abandoned 19th-century alum shale mine. The water exhibits a distinct reddish-brown hue caused by high concentrations of iron sulfates and vitriol. Due to its extreme acidity, typically between pH 2 and 3, the lake remains completely devoid of aquatic life and plant growth. The site is a protected geological monument highlighting historical mining practices in the Plzeň region. Steep, unstable slopes surround the water, preventing direct access to the shoreline for safety reasons. The surrounding environment consists of a forest landscape that reclaimed the industrial excavation area over the last century.
The highest cliff edge looking down over the lake provides the clearest view of the water's distinct red color.
View the lake only from the designated observation points located on the upper rim.
Wear sturdy footwear suitable for uneven forest terrain and sandy paths.
Bring mosquito repellent, as the surrounding forest areas are often heavily infested during summer months.
Do not touch the water or the damp soil near the shore, as the acidic properties can cause skin irritation.
Do not attempt to descend the steep, unstable banks to reach the water's edge; stay on marked paths.
