  We study Kustin-Miller unprojections between Calabi-Yau threefolds or more
precisely the geometric transitions they induce. We use them to connect many
families of Calabi-Yau threefolds with Picard number one to the web of Calabi
Yau complete intersections. This enables us to find explicit description of a
few known families of Calabi-Yau threefolds in terms of equations. Moreover we
find two new examples of Calabi-Yau threefolds with Picard group of rank one,
described by Pfaffian equations in weighted projective spaces.
