Karlstrup Kalkgrav, the limestone quarry located in Solrød, is considered Zealand’s clearest lake. The area is popular in the summer when everyone enjoys picnics near magnificent azure waters.
The quarry started operating in 1843 and was in use until 1975, after which a lake emerged. During that period, many houses around Copenhagen were built with stones from Karlstrup.
Currently, the lake is approx. 6 ha up and 14 m deep. The trip around the lake is around 2 km and can be done in about 45 minutes. However, it will probably take longer, as you can stop and try to look for well-preserved fossils of animals that lived on the bottom of the sea about 55 to 65 million years ago that are abundant in the area.
You can find fossils of sea urchins, sea mice, corals, or shark teeth in the area. More info in danish in this document.
It might be helpful to bring a small hammer with you to loosen up the rocks.
Even though the lake is crystal clear and might be tempting just to jump in, it is not recommended to swim in it as the water is ice cold just below its surface.
It is possible to dive in the lake; however, your diving must be booked and approved by the Danish Nature Agency.
Check more info under this link.