One of our outings was a trip to Lac Pavin, which is a meromictic volcanic crater lake. In other words, there are effectively differnt layers of water in it which rarely mix, maybe not even for decades or centuries, resulting in quite different environments at each water level. In most lakes, the surface and deeper water levels intermix at least once a year.
The volcanic crater is approximately 1000 metres in diameter, and is filled by the lake, which is 750 metres in diamter and 93 metres deep.
We nearly didn't go on this trip, as it was very foggy even when we left the campsite, and as we drove higher towards the lake, it got mistier and mistier...but we persevered. With the brighter summer evenings, I think this was the only time we needed to use the car headlights.
It was a little under a five-mile walk from the cafe at the car park, up the side of the mountain to the summit of Puy de Montchal, down and up through a small crater and back down the far side of the lake - by which time the sun was really starting to break through.
At the summit, we had a lovely chat with a group of French walkers who, like us, were stopping for a a snack before continuing their trek. The path we were following was very well way-marked, and while we needed to consult signs on a couple of occasions, we never thought we were at risk of getting lost.