I collected the locs of the different plane portals in PoT, some I have no idea where they lead :)
Plane of Nightmare
Between some buildings
-184.70, -1296.74
Plane of Fire
244.57, -2194.73
Plane of Disease
1664.00, -1389.42
Plane of Innovation
-1556.04, -16.87
Plane of Justice
59.64, -651.22
Plane of Earth
-1374.53, 2144.42
Plane of Water?
-1182.61, 1003.27
Plane of Air? Elemental?
868.25, -699.79
Unkown Plane
712.99, -1661.13
Tunnel to Unknown Plane
1894.21, -1054.60
Tunnel Leading to Two Unknown Portals
-992.66, -204.21
Inside a buliding / slash tunnel on top of hill
Leads to what looks like several different portal stones
-191.98, -16.82
Inter-Planar Portal
-2226.64, 1216.70