Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
A tunnel in a dream indicates cunning and deceit or a hidden matter. Entering it indicates entering hardship. Exiting it indicates salvation and relief.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
A tunnel indicates prison or the grave. A dark tunnel indicates worry, grief, and misguidance.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
A tunnel is interpreted as worry. Whoever sees himself enter it will be afflicted with worry. Exiting it is relief.