It is still in rainy season here in Mt. Fuji area. When it rains really heavily, you can have rare findings in the forest such as small ponds(!) on the ground, sounds of water running(!) through the cracks of lava rocks. But, they will not last long and majority of the water eventually seeps down into and through the ground which is lava.

So, inside lava caves in rainy season, sometimes rain leaks badly through the lava and I can see visitors to e.g. Fugaku Wind Cave using even umbrella as they walk through it.

Aokigahara under the rain