'Natural' Dungeons (not just caves!)- any ideas on making them interesting?

[pogre]

One other "dungeon-like" option might be cliff dwellings, like those at Mesa Verde.

 

[Fast Learner]

A swamp with lots of bubbling tar and hanging vines would work well. Flying creatures would be somewhat limited, but walking creatures would need to follow existing sandy paths to avoid be trapped by the goo.

Canyonlands provide a rich, dungeon-like structure with plenty of places for lairs and such.

 

[Dreaddisease]

Also since D&D contains large creatures and monsters that drill through the earth you can use that.

A large purple worm traveled through an area once awhile ago and residents have built supports to keep the ceiling from falling. Or some creatures drilled through rock, maybe to hibernate for a couple years. These are ideas for straight lines or simi-wavy caves.

The natural forming caves would always be much more complex and weave in and out constantly with pools and what not.

 

[Vuron]

Besides the tunneling effects of Purple Worms, Delvers, etc there is also room for magical effects clearing out large areas such as elemental rifts where link to the plane of air creates a large space underground. Or perhaps a magical catastrophe forced whole locale deep under the earth.

 

[the Jester]

If you can find a copy, check out the old 1e Dugeoneers Survival Guide hardbook. It talked extensively about natural caves, how they form, features, hazards, water dangers, etc.