Galaxy clusters as astronomical tools generally offer two advantages: to boost the brightnesses of objects in the background, and to study the dark matter in the lens. We can now introduce a third use: to unveil properties of the underlying dark matter by the detection of caustic crossing events which can yield magnification factors of 10,000 or more. We review the approach to detect a new set of massive objects such as galaxy clusters by their rest-frame far-infrared colors (and not by the Sunyaev-Z'eldovich effect). We move on to present Hubble Space Telescope WFC3-IR imaging in the fields of six classical giant arcs discovered using a combination of Planck and Herschel space facilities. We construct lensing models from the many examples of image multiplicities, and compare our results with galaxy over-densities identified by more conventional methods. We conclude with a discussion on the emerging field of caustic crossings.