The terahertz spectral regime, ranging from about 0.1–15 THz, is one of the least explored yet most technologically transformative spectral regions. One current challenge is to develop efficient and compact terahertz emitters/detectors with a broadband and gapless spectrum that can be tailored for various pump photon energies. Here we demonstrate efficient single-cycle broadband THz generation, ranging from about 0.1–4 THz, from a thin layer of split-ring resonators with few tens of nanometers thickness by pumping at the telecommunications wavelength of 1.5 mm (200 THz). The terahertz emission arises from exciting the magnetic-dipole resonance of the split-ring resonators and quickly decreases under off-resonance pumping. This, together with pump polarization dependence and power scaling of the terahertz emission, identifies the role of optically induced nonlinear currents in split-ring resonators. We also reveal a giant sheet nonlinear susceptibility ~10-16 m2V-1 that far exceeds thin films and bulk non-centrosymmetric materials.