Instructions
Complete level 8-1 to gain access to level 8-2, which contains the secret exit leading to level 8-7.
Progress through level 8-2 and go down the pipe next to an arrow sign at the dead end. You will appear underground in an area with numerous large spinning wheels you must run across.
Go past the halfway point in the level and continue right until you jump over two horizontal yellow and red pipes with goombas coming out of them. Follow the rolling spiked balls a little further right until they drop down onto several rolling wheels. Drop down and run left through the wall to find a pipe leading to the level's secret exit.