Abstract
A new deep imaging survey of galaxy clusters aims to find the most powerful gravitational lenses in the large Gigaparsec3 volume of the Sloan Digital Sky Survey (SDSS). So far, this ongoing survey has deeply imaged nearly 500 massive clusters, mostly using the Nordic Optical Telescope, and has revealed some of the most dramatic examples of gravitational lensing ever discovered. Follow-up spectroscopy and imaging at the Subaru and Gemini telescopes seek to address whether the dark matter distribution in galaxy clusters is consistent with predictions in the currently favoured LambdaCDM cosmological model. Our first results reveal strongly significant discrepancies between model predictions and observations. In addition, these cluster lenses can be used as "gravitational telescopes" to study the high-redshift universe. Finally, I will describe the relevance of this cluster survey for the Planck mission.