1. Silfra Fissure is a unique geological formation located in Thingvellir National Park near Reykjavik, Iceland. It is a narrow crack between the North American and Eurasian tectonic plates, which are gradually moving apart from each other at a rate of about 2.5 cm per year. Silfra is filled with crystal clear, icy cold water that has been filtered through the volcanic rocks for decades, making it one of the clearest and purest freshwater environments in the world. The visibility in Silfra can reach up to 100 meters, allowing for a stunning natural view of the underwater landscape. Snorkeling or diving in Silfra is an amazing experience for many adventure seekers.

2. The site is open daily throughout the year, but reservations are required and can be made through various tour operators. The working hours and phone numbers of the tour operators can vary, depending on the season and the specific tour package. Some tour operators to consider are Dive.is, Iceland Adventure Tours, and Arctic Adventures.

3. The cost of snorkeling or diving in Silfra is typically included in a tour package, which can vary depending on the type and length of the tour. The price can range from around $150 to $250 USD. Transportation to Silfra is also included in some tour packages, but visitors can also rent a car or take a bus from Reykjavik to Thingvellir National Park. Parking is available at the park for a small fee.