Granada Hoy outlines that the study for the Expansion of the Granada Metro lists 7 lines of advantages and 32 lines of disadvantages. "Severe construction risks" make the construction of 1,5km long tunnel under the historic center of Granada almost impossible. The tunnel is part of option C2 of the Metro line expansion.  The tunnel would have to pass through an area with archeological remains between 2-10 meter depth. One of the other problems is the difficulty of accurately locating the underground channel of the Darro river. 

The tunnel option would cost 223 million euros, 89 more than the over ground option.

https://www.granadahoy.com/granada/Dificultades-extremas-Metro-Centro-Granada_0_1639036172.html