The Gate to Alteria
Alteria and Earth are connected by a gate in the middle of the ocean [can’t disclose exact location].
The celestial entities of each world are visible in the other, and the gate is powered by the celestial structure of the location it is leading to – specifically the sun. This allows for a one-sided closing of the gate by preventing from enough sun rays hitting the surface.
This is how the keepers keep the gate to Alteria closed – they partially block the Alterian sun from hitting Earth’s surface. The blockage provided by a single keeper is enough to close the gate, therefore all five keepers must relent fo the gate to be opened.
The gate washes off and substance and anything altered becomes unaltered when it crosses from Alteria to Earth. The only time someone had successfully brought anything substanced or altered into Earth without it “washing off” was the substanced water Everett and Carolyn Clark brought when they built the forts.
Alterian keep their side of the gate closed for security purposes, although it is very unlikely anyone will cross willfully and remove any Alterations they enjoy.