Hyvinkää Geochallenge is half of the dual cache system, the other part is Riihimäki Geochallenge. Both caches can be logged separately, but in order to find them, you have to collect information from the both cache pages.
Finding this cache system starts with a mathematical homework. The cache twins are defined by two points on the highway #3 from Helsinki to Tampere. Hyvinkää reference point is N 60° 39.043, E 24° 48.172. The distance from this point to both cache locations is equal, and the distance between the cache locations is 1029 meters.
The plastic container is placed close to a strange exploded cave on top of steep cliffs somewhere in the Usmi recreation area. But please note: The cache is no longer inside that cave, so you don't have to do anything even potentially dangerous to find this cache. The calculations above lead still to that cave.
So, you have found the cave... Now choose a safe direction and take maybe a little more than 15 steps towards that direction. Can you now see a bird box both on the left and right sides of you? If yes, you are very close to the cache. Look under the most suspicious looking stone.