That is literally the tunnel into the Altausee salt mine that thousands of visitors walk through regularly (including me a few years ago). 

https://salzwelten.at/en

You can see a picture here
https://salzwelten.at/Medien/Altaussee/38/image-thumb__38__lightbox/SW_AA_gefalltete_Steinsalzschichten_mit_Stolleneingang.jpg