The last generation of micromegas, called microbulk, are ahead of classical gas detectors (or even other kind of micro-pattern gas detectors) in gain stability, efficiency (by operation at high pressure), simplicity, robustness, energy resolution, readout features and radiopurity. This makes them a competent solution in the field of Rare Event Searches, a field where low background is the most appreciated feature of a detector. The CAST (CERN Axion Solar Telescope) experiment is the best example of their application in the X-rays range. In CAST, these detectors have achieved background rates as low as 6 × 10−6 counts keV−1 cm−2 s−1. Beyond this nominal operation, there have been several periods where the background has been reduced to a level of 2 × 10−7 counts keV−1 cm−2 s−1, due to reasons which are under investigation. The CAST experiment will be presented, paying special attention to their microbulk micromegas, as well as the procedures to achieve low background. Latest news about the operation of these kinds of detectors for the first time in underground conditions will be advanced here. Copyright © 2011 John Wiley & Sons, Ltd.