The VAPOR project at NCAR aims to provide interactive 3D visualization and analysis for the earth science community.  One of the greatest obstacles to data analysis and visualization is the recent expansion of computed data sizes, currently toward petabytes, and soon to exabytes.  Reading a full data set from disk can itself take many days.  VAPOR's approach is to make such datasets understandable by accessing the data from a wavelet representation, and then by exploiting the graphics card performance with interactive 3D graphics.  In this framework it is essential to provide a highly interactive and intuitive user interface.  This talk presents an overview of the capabilities of VAPOR and the underlying architecture, together with a discussion of some of the interesting software engineering challenges we continue to face.