Learn Finnish: Lämpötila nousee sitä mukaa kun louhimme kallioperää. - The temperature rises as we excavate the bedrock.

While kun simply means 'when' or 'as', sitä mukaa kun specifically highlights a proportional progression. It emphasizes that the temperature rises in direct proportion to the excavation: the more they dig, the hotter it gets.

Why is the word mukaa used here instead of the standard postposition mukaan?

The phrase sitä mukaa kun is a fixed expression. The word mukaa is an old, fossilized form related to mukaan. You don't need to decline it or add an 'n' to it; just memorize the entire phrase as a single set block.

Could I use a more common verb like kaivaa (to dig) instead of louhimme here?

You could, but louhia is much more precise for this context. While kaivaa means to dig generally (like in dirt), louhia specifically means to excavate, mine, or quarry solid rock. Since the object is kallioperä (bedrock), louhia is the most natural verb choice.

Why is kallioperää in the partitive case here?

It is in the partitive because the action of excavating is ongoing and unbounded. They are in the continuous process of mining the bedrock, rather than having finished excavating a specific, complete chunk of it.