  One of the key ingredients of the Unified Model of Active Galactic Nuclei
(AGN) is the presence of a torus-like optically thick medium composed by dust
and gas around the putative supermassive black hole. However, the structure,
size and composition of this circumnuclear medium are still matter of debate.
To this end, the search for column density variations through X-ray monitoring
on different timescales (months, weeks and few days) is fundamental to
constrain size, kinematics and location of the X-ray absorber(s). Here we
describe our project of mining the Swift-XRT archive to assemble a sample of
AGN with extreme column density variability and determining the physical
properties of the X-ray absorber(s). We also present the results obtained from
a daily-weekly Swift-XRT follow-up monitoring recently performed on one of the
most interesting new candidates for variability discovered so far, Mrk 915.
