If planeswalkers were moths, Zendikar would be a dazzling, fiery light.
All of Zendikar is dangerous. The world seems almost as though it's trying to kill its own denizens, whether with monsters, natural hazards, or traps laid for the unsuspecting. Everything on the plane is precarious, unpredictable, or just plain lethal. The world seems dead-set on protecting its unique treasures—both the literal ones and the most prized, most ephemeral one: its mana.
Like other planes, Zendikar's lands flow with mana that mages can use to power their spells. However, Zendikar houses a "primal" mana. This spell-like mana seems almost alive to those who wield it. It has caused Zendikar to be a dynamic world crackling with intense magical effects. Sometimes the sea blasts forth geysers of elemental water that form floating islands; the peaks of mountains lurch up and down to crush those who would scale their heights; forests alter their own flow of gravity or patterns of growth.
To planeswalkers, this unique mana is an irresistible prize. To most of Zendikar's people, planeswalkers are foolhardy, power-hungry creatures who will risk life and limb for an elusive, unreliable prize.
Large, mysterious, stone hedrons litter the plane. They are remnants of a strange and ancient civilization that wielded unimaginable arcane power—enough to suspend gravity, to upheave the land itself, and to change the plane's life to suit its purposes. But long ago that civilization collapsed for reasons few know. Now these crumbling remains are scattered across Zendikar—some buried in the land, some slowly wearing away on the surface, and some still hanging in the sky. These ruins and artifacts still emanate power, although most denizens of the plane know better than to disturb them.
Zendikar's unique mana, the hedrons, and its own fierce ecology all combine to form erratic terrain subject to sudden, violent changes. The land itself seems alive, and its surface and botanical life sometimes writhe as though in pain, causing tectonic chaos, extreme weather, and abrupt destruction. All this volatility is collectively referred to as "The Roil."
For the sentient creatures of Zendikar, The Roil is simply a natural phenomenon—the way things are. To planeswalkers, it's obvious that this volatility is what keeps the plane dangerous and wild, free of large cities, sophisticated commerce, and other trappings of well-developed civilization. Zendikar is untamed ... and perhaps untamable.