At the end go into the tubes while pessing the button between the left and down direction. You'll go into a room with spikes at the top and an emerald. On the 2nd slope run down and bounce on a set of trampolines. Hold right until you hit another set. Then hold right for about 2 seconds.
Gimmick mountain Go to the part where the badnick is at.
Kill it and jump through the wall. Get the other life and the wheel. Run right until you find the emerald. For every 2nd act on Sonic 2, there are secret crystals that will get you 3 extra lives. You usually have to find them between gnooks and crannies.