Helsinki centre is hollow like a Swiss cheese. A great deal of the underground space is reserved for parking, but there are also many underground pedestrian connections and shopping malls. The Railway station, shopping malls Forum, Kamppi and City-Center and department store Stockmann are all connected to each other. This underground network extends From Kamppi all the way to Kluuvi and Kaisaniemi, but this connection is for service traffic only. If you go under ground here, you can (in theory) reach geocaches GC23VEY, GC24FKH, GC2G8CH, and some others too without popping up on the ground earlier than about 50 meters from the cache. But unfortunately pedestrians are not allowed to enter here. The cache is not under ground.
This time you do not have to calculate anything! On a contrary, you will find a spoiler for HUC #1 (GC23VEY) from this cache. #1 is only 480 meters from here.
The location is very public and the GPS coverage is quite poor at the site. I apologize your inconvenience... Try to figure out where a cache of about one litre could fit. There are not many such places here! The encrypted hint has two levels, the other on is quite a spoiler. Try some searching first.