I went to check out a place for downhill riding. I think the best route is in the forest. It's the nearly straight path seen on the far left in the Sports-Tracker excercise. It has some curves, and it has only a few small rocks. OTOH, I did not see clearly every obstacle because of the stuttering (no rear suspension) and wet eyes ;) The bicycle meter recorded maximum speed of 35 km/h. It really felt a bit faster! Not once before I've needed to hold so tight on the handle bar. That was interesting!
Besides writing something, I thought it's about time to try how embedding the excercise works (with the new layout, it doesn't seem to work):
Open the workout on Sports-Tracker website.
Anyway, the hills have some / lot of vegetation, which makes fast riding very dangerous, as there might be (there is) some rocks or potholes you can't see. To make it clear: I only tried riding where I could see the ground. I also noticed that just going down is not so rewarding. There should be a trail with bends and some jumps and stuff. As it is now, Hasamäki offers very little for downhill riding. But, if there would be a group of DH riders interested making it a DH-able place, it is possible.