Achieving high-Q resonances in the THz frequency range is significant for applications such as sensors, filters, and emitters. A promising approach for obtaining such resonances is by using metamaterials. However, high-Q resonances in THz metamaterials are usually limited by metallic radiation losses in the meta-atoms. In this Letter, we investigate both experimentally and numerically a complementary metallic disk-hole array (CMA) that uses the coupling between lattice resonances and Fabry-Pérot cavity resonances, and features in-substrate modes with experimentally obtained record breaking Q-factors of up to 750. To the best of our knowledge, this is the highest quality factor measured for free-space-coupled metallic metamaterial structure at THz frequencies.