Seismic noise, the related gravitational gradient noise (Newtonian noise) and the thermal noise of the suspension last stage and of the test masses determine the sensitivity of current gravitational-wave detectors at low frequencies. Seismic and Newtonian noises can be mitigated by installing the detectors under the ground, while the thermal noise can be reduced by cryogenically cooling down the test masses. One of the proposed subterranean detectors, the European Einstein Telescope (ET) is planned to be built 200 to 300 m below the surface. The noise budget of ET at low frequencies is expected to be dominated by seismic noise and Newtonian noise of seismic and infrasonic origin. In order to get to know the long-term evolution of these types of noises, the Mátra Gravitational and Geophysical Laboratory (MGGL) was built 88 m below the surface at the Hungarian Mátra Mountain Range. Infrasound measurements and seismic noise measurements were started in 2017. The seismic noise measurements are still going on. We investigate the evolution of the Newtonian noise via applying the most recent modelling methods presented in literature and using our measurement data as input. I give an overview of the measurements done at MGGL, the modelling methods of Newtonian noise, and the results of our investigations.