One begins on a high ledge at the very beginning and so is only accessible by use of Tails or Knuckles (see Point #1 below).

Act 1 has two main alternative routes in addition to the regular one, but their entrances are not quite as obvious to find. If you're moving fast enough, you can actually sprint right across the surface of the water without falling in, in some areas. Remember that your character runs out of air and will drown after 30 seconds, so look out for small bubble clusters that produce large air bubbles, which you can suck up to fill those lungs again. While your trips underwater aren't as long as they have been in past levels, you'll want to keep an eye out for air bubbles to keep you going. You can find some of this kind of thing underwater as well, but it's mostly more slow-paced down there, with more tricky formations of objects and small platform-based puzzles to contend with, based on straighter paths with steps. Quite often there are two paths leading into a loop, and you'll enter it through one of them, then hit a spring at the end of the loop that sends you back through it, and you'll exit on the other path, so there's a lot of back-and-forth action there, but you're generally going mostly to the right all the way through.

Above it, the area is crammed with loads of loop-de-loops and twisting, sloping paths. Act 1 is generally based in a number of fairly large rooms within the setting, and a few smaller corridors under the water.

Sonic windows zone act 2