Initailly named “Carol I Tunnel”, then known as Palas – Port C-ta passage, the railway tunnel functioned as an freight trains access to Constanta harbor for almost a century.
These days the tunnel is no longer in use. It was rehabilitated in 2012, and finally closed for normal visitors. More to read, with plenty of pictures on following link: Visit TheTunnel
Last minute update : There is no way to reach up to the Tunnel these days. But you can reach as far as to the cache location, find the Container I hide and see the top of the Saligny Tunnel - you will be a few meters away of it (the other side of the fence).