Zen is sometimes described as “the mountain still state”, and we are often admonished to sit like a mountain. Monasteries were frequently named after mountains; Teachers too.
At the most obvious level, the mountain can be seen as representative of equanimity, imperturbability. Whatever storm is raging, the mountain is undisturbed.
We can also see the mountain as the expression of something eternal. So, when we enter the mountain still state, we enter the same state as the ancestors and patriarchs.
But fundamentally, the mountain is the ground made visible, unavoidable. Whilst the ground beneath the feet of our thoughts is overlooked, the mountain is the ground thrown upwards. And the ground is being.