Sand Hill (サンドヒル,Sando Hiru?) is a stage only playable in Tails' story in Sonic Adventure. In this stage, Tails has to pass through the various gates to earn points.
After getting blasted out of the sky at the end of the first Sky Chase by the Egg Carrier, Tails and Sonic were separated when the Tornado crashed. After having a flashback about the "good old days", Tails realizes he can't rely on Sonic forever and figures that he should try the Tornado II but it needs a Chaos Emerald to work. Tails happens to find a Chaos Emerald in the jungle section of the Mystic Ruins but it is swallowed by Big's best friend, Froggy. Tails gives chase, which winds him up in Sand Hill. After catching the frog and retrieving the Emerald, Tails is transported into the past by a ball of light. When he returned to the present, Tails use the Chaos Emerald to finish building the Tornado II.
In Sub Game, this level can only be played by Sonic and Tails. You can receive two emblems from this stage, (like other sub games). You get the emblems by receiving high scores. The second emblem is much harder than the first. You can use either Sonic or Tails, but cannot get two emblems per character.
There is a very large possibility that Sand Hill actually takes place in Sandopolis Zone from Sonic & Knuckles (or Sonic 3 and Knuckles when Lock-On is enabled). This is heavily supported by the fact that the entrance to Sand Hill is very close to (if not actually part of) Angel Island.