The present study is focused on the dynamics of a microbeam-based MEMS device and analyzes its behavior in the neighborhood of the third natural frequency. An extensive experimental investigation is conducted. The main resonant and non-resonant branches span a wide range of coexistence. The 2:1 internal resonance is activated between the third and fifth modes, in which case the device exhibits complex and intriguing dynamics. The experimental data are examined in depth using various analytical and numerical tools. Alongside with the experiments, theoretical simulations are developed, where the main features of the internal resonance are properly represented and the contribution of each mode is discussed. The main steps of the progression of the 2:1 internal resonance are highlighted and the possibility of more complex internal resonances is explored, where different higher modes are involved.