A new generation of accelerometers based on a microelectromechanical system (MEMS) can deliver broadband (0 to 800 Hz) and high-fidelity measurements of ground motion even at a low level. Such performance has been obtained by using a closed-loop configuration and a careful design which greatly mitigates all internal mechanical and electronic noise sources. To improve the signal-to-noise ratio toward the low frequencies at which instrument noise increases, a new MEMS has been developed. It provides a significantly lower noise floor (at least −10 dB) and thus a higher dynamic range (+10 dB). This performance has been tested in an underground facility where conditions approach the minimum terrestrial noise level. This new MEMS sensor will even further improve the detection of low frequencies and of weak signals such as those that come from deep targets or from microseismic events.