This cache is an adventure lab bonus. To discover the coordinates, you must first complete all stages of the 'Vierumäki: a visitor's guide' tour.
The cache itself is located in a scenic area on the east side of Suurijärvi. You can consider this the sixth stage of your tour, during which you will see some of the numerous cottages and outdoor recreation areas available. If it's summer, you may want to consider taking swimming gear with you when retrieving this cache.
Cache location:
N61 07.ABC
E26 00.DBE