...The ocean is entirely under the North Pole...
To build a decent military base, and not just one that everyone has, at least three steps are necessary.
1. You need to break through the ice first, just use an ice shaver. If it is intended for commercial use, it cannot be intended for domestic use.
2. Then, if you want to dig dirt, buy a tomb robbing novel and see how the native master digs it. You can do the same.
3. Finally, build a military base in the cave. Be extravagant and thrifty, but pay attention to size, layout and color matching. There is no harm in building a few more toilets. During the most tense period of the war, many people must have urgently needed to urinate.
Then keep it well. A military base is not a toy. You can't throw it awaywhen you grow it and think it's not cute. You must be responsible for it.