What does 適度な所で斜め上に向かって穴を掘る mean here?


> 出る時は斜めに掘る必要があるな。いや、待て待て。
>
> 縦の穴。酸素はどうなっている？ マズイ、死ぬ？ 俺は慌てて斜め上に向かって穴を掘り、脱出する。
>
> 反省。次は空気の通り道を考えながら、掘ってみた。
>
> 基本、斜め掘り。適度な所で斜め上に向かって穴を掘る。俺の中のイメージの井戸とは全然違うが、要は空気が流れればよい。
>
> 最終的に下に向かって掘っていれば問題はないだろう。

So, a guy tries to dig a well but figures out that if he dug vertically it
might affect the oxygen level, so he decided to dig diagonally. What confuses
me is this 「適度な所で斜め上に向かって穴を掘る」? Does it mean he stops at a certain place, digs
diagonally upward​, and then proceeds to dig horizontally? Or that he digs
diagonally all the way, but occasionally digs holes on his way diagonally
upward​ (this is how a fan translation has it)?




Simply digging a long zigzag hole obviously won't provide oxygen, so if I
understand correctly, he is doing something like this:

[![enter image description
here](https://i.stack.imgur.com/cbdLo.png)](https://i.stack.imgur.com/cbdLo.png)

The final result is a hole with some "branches" to serve as [ventilation
raises](https://en.wikipedia.org/wiki/Underground_mine_ventilation).

