For many years all who traveled to the frozen wastes of Barsoom's northern pole would never return. This is thanks to the "guardian of the north." The object appears as a gigantic black obelisk, near the city of Kadabra. It is a giant electromagnet situated directly over mars' magnetic north pole, though it is unknown if this has any actual effect on the magnet.
The magnet when activated draws fliers near it, causing them to crash into the obelisk. Those who survive are gathered and become slaves of the yellow men. The guardian is controlled by a a black lever with a white symbol inlaid upon it. This lever was controlled by an old man named Solan, until the Warlord of Mars killed him and thus liberated the frozen north.