Is mountain climbing your hobby?
I went to climb a mountain with my quiet friend.
Both of us have been through a lot, but we had a long talk while climbing a mountain and decided to get back together.
You've hopefully seen how 登る means "to climb." So, if you're climbing a mountain what is that? It's mountain climbing.
The reading comes from combining 山 and 登る together to form one word, 山登り. If you know the readings of those two words you know the reading of this word too.