Gerede tunnel is no spectacular. It is only a water tunnel 5,5m diameter. 31.6km long.

Türkiye has more interesting tunnel projects. For example Bosphorus tunnel connecting Europe to Asia.

But Gerede is more challenging because geology is very dificult.

This news says "The team working on the tunnel had to overcome a total of 48 major fault zones and water pressure build ups of up to 26 bar." 

"Robbins XRE TBM was called in to complete the tunnel, after three Double Shield TBMs from another manufacturer had encountered incredibly difficult geology including massive inrushes of mud and water."

What manufacturer'sTBMs that were not able to do the job? I don"t know but i will investigate and write here

you can read it: Tunnelling: Breaking new ground