If you take the base pad and chip away you can sometimes get thru. but if they are tall mountains then you are out of luck.

I found you can by putting mines close together, but not if too big. I then transfered all my non O2 buildings on other side but also had to plan another mine to mine transfer point just in case.

Using a combination of a base pad, and connecting a mine to it but as far into the mountain as you can will often clearly out entire areas. But as Donnie said, if it's too tall, out of luck.

IF i get around to it i'll try post a screen shot. I've found though that sometimes building through mountains and not doing it right has caused pathing issues. Mostly if there is an airlock and mine in an area that was once a huge mountain. Doesn't happen all the time, but sometimes.

hope this helps: http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=723475139

The devs should really fix this issue - I've spent hours on a base only to discover I'm completely boxed in by mountains :(

i spawned in and there wasnt room to even build a basic base well that sucks lol