If I were to mine netherrack upwards or downwards, left or right in one direction, would I hit bedrock, or would I keep on going? What would happen?
From Minecraft Wiki's page on The Nether:
... Under the Lava ocean, there is a layer of Nettherack1 and Bedrock on the very bottom. Similarly, the very top of the Nether is Bedrock.
1 Or Bloodstone
Basically, going up or down either way leads to Bedrock.