Seen from above, the forest on the north side of Mount Fuji looks like green felt, and underneath it lies black lava from an eruption in 864 AD. The lava poured through the trees, the wet trunks cooled it around them, and when the timber burned or rotted away it left hollow cylinders in the rock. Those are the tree molds at Funatsu, a short walk of mossy, cool tunnels where you can duck into the lava and look at the exact shape of a tree that stopped existing eleven hundred years ago. Some of the molds are the width of a person's shoulders; a few go deep enough that you want a torch.
The same eruption dammed the old lake and split it into smaller ones, which is why the Fuji Five Lakes exist at all. Aokigahara, the dense forest growing on that lava, is a short ride from here; the ground is porous and the compass needles behave oddly, and the whole thing smells of damp earth and cedar. It is quiet in a very particular way, because the lava soaks up sound like a sponge.
Getting here is half the fun. From Fujiyoshida, a proper town of nearly fifty thousand people with every kind of fuel station, convenience store and ramen counter, you climb through pine woods and the riding turns to long smooth bends that suit a loaded tourer. The Fuji Subaru Line toll road climbs the mountain's flank toward the fifth station, but it is well known that the road is closed to private traffic at times in summer, so check before you plan on it. The better lakeside roads around Kawaguchiko run past thatched farm houses and offer the view that ends up on postcards: Fuji reflected in glassy water, if the cloud allows.
Weather decides everything. Fuji hides behind cloud more often than not, and the best views are in the cold clear mornings of late autumn and winter. The air up here is around a kilometre above sea level, so nights can be sharp even in May. Summers fill the roads with tour buses and rental cars, and the lakes get busy at weekends; a weekday and an early start gets you the quiet version.
Do not skip the local food: houtou, the thick miso noodle stew with pumpkin that Yamanashi keeps in a pot, is exactly what you want after a morning in the damp tunnels.