I need to create a world with some very high plateaus and some very deep depressions. The sides of these structures should be very steep and unclimbable. They need to provide an obstacle sufficient to ensure that the the plateaus and depressions are only accessible from the surface by air transport.
The heights and depths also need to be sufficient to ensure that the climates are very different from the surface, but most areas should still be habitable when reached, ideally 3-6 miles above/below the mean surface elevation. The plateaus should be of the order of 100 miles across; the depressions (craters?) should be of the order of 1000 miles across.
Assume an approximately earth like world except with much less water. The depressions must not simply be oceans, although some water may be present. Other than that any physical characteristics can be adjusted if this helps. If this is not possible what comes closest? No magic.