Act 2 introduces more complex platforming and environmental hazards. You'll encounter more Choppers (piranha-like robots in water) and Newtrons (chameleon robots). Be wary of the collapsing platforms; they will fall shortly after Sonic steps on them. Use your speed to cross them quickly. There are several underwater sections; remember Sonic can only hold his breath for a limited time before drowning. Look for air bubbles to replenish your oxygen.
A notable secret in this Act is another 1-Up Monitor. After a series of collapsing platforms and a large loop, you'll come to a section with a spring that launches you high. Instead of going straight, try to land on a small, hidden platform to the left, which contains the extra life. This Act also features more opportunities for high-speed sections, so try to maintain your momentum through the loops and slopes to reach the goal faster.