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If the room above has a narrow sand exit, then spin jump through the transition pressing against the wall, continue holding left to reach the wall above the tunnel, then perform a tight mid-air morph and hold left to pass through the tunnel. If Samus falls below the tunnel, even to the ledge just below, it will no longer be possible to reach.

Entrance condition:

{
  "comeInNormally": {},
  "comesThroughToilet": "no"
}

Requires:

{
  "notable": "Pass Through Hidden Tunnel"
}
"canPrepareForNextRoom"
"can4HighMidAirMorph"
{
  "enemyDamage": {
    "enemy": "Bull",
    "type": "contact",
    "hits": 1
  }
}
{
  "or": [
    {
      "enemyDamage": {
        "enemy": "Bull",
        "type": "contact",
        "hits": 1
      }
    },
    "h_canUsePowerBombs"
  ]
}

Dev note: FIXME: Depending on the room above, the 'can4HighMidAirMorph' requirement might be unnecessary. FIXME: Entering through the Toilet can be possible with a lateral mid-air and enough run speed from an air room above.