This subterranean lake has a length of around 300 meters, which they say makes this the largest natural underground lake in Europe. Even better it can be explored during a 30 minute boat ride. Most of the guides are able to speak in English, so they translated sections of the tour to those of us on the row boat that were English speaking. The lake is open the 19th of March to the 1st of November. You can take the train easily to St. Leonard and then walk up. At the entrance to the lake is a small cafe and trinket tourist shop, where you can purchase your ticket for 10 CHF. In Switzerland, very few places have indoor air conditioning, so it is hard to find a place to get out of the heat to relax. One extremely hot day, we decided to check out the underground lake. As you walk down the stairs to get onto the boat you can feel the temperature drop significantly. The guide told us that no matter what time of the year the temperature remains the same down there. It was amazing to be on a boat in a cave, you feel like you are in a movie. The water is so clear you can see to the bottom. It was beautiful. The 10 CHF was definitely worth the 30 minute retreat from the hot heat. We almost went on the boat a second time because it was so enjoyable. They have fish that they placed into the lake to keep the water clean. There were HUGE. You could see them swarming all around the boat. read more