  Systematic exploration of the observable parameter space, covered by large
digital sky surveys spanning a range of wavelengths, will be one of the primary
modes of research with a Virtual Observatory (VO). This will include searches
for rare, unusual, or even previously unknown types of astronomical objects and
phenomena, e.g. as outliers in some parameter space of measured properties,
both in the catalog and image domains. Examples from current surveys include
high-redshift quasars, type-2 quasars, brown dwarfs, and a small number of
objects with puzzling spectra. Opening of the time domain will be especially
interesting in this regard. Data-mining tools such as unsupervised clustering
techniques will be essential in this task, and should become an important part
of the VO toolkit.
