Level 20 is the twentieth level of the Main Tunnel.
Oh wow, is that a spiral level?
For this level, we highly recommend using the Skater as you can clear the level in one jump. No, seriously. One jump.
When you start, change gravity to the right. Then when you get to the crumbling tile, jump. Then immediately move over to the right.
But with the Runner, it's harder to do that as you have to move left in midair (changing gravity in midair). You can also do this: Jump, move right, and repeat.
And now the tunnel changes color. It's turning green.
This level references level 20 in run 1, in fact.