On 19 September 2021, in La Palma, Canary Islands (Spain), a new volcano, later named Tajogaite, erupted. We determined the shallow seismic tomography model of Tajogaite Volcano, including the 3D distributions of Vp, Vs, and Vp/Vs. The tomography of the volcanic edifice has been performed using a combination of a three component amplitude method for seismic event detection and a sophisticated deep neural network method for picking first arrival times, which allowed the detection and the picking of thousands of earthquakes from a data set recorded by a temporary network deployed around the main crater just a few months after the end of the eruption. We identified shallow low Vp/Vs and deeper high Vp/Vs anomalies, which we interpret as changes in the fluid filling the rocks, transitioning from liquid to steam at shallower low pressure conditions. Our work highlights, for the first time, the internal structure of a monogenetic volcano, specifically its evolving local hydrothermal system, right after its eruption.