The E39 under the Mastrafjord in Rogaland runs 4.4 kilometres between Askje and Rennesøy. Together with the Byfjord tunnel it forms Rennfast, the fixed link between Stavanger and Rennesøy.

The tunnel opened on 30 November 1992, the same day as the Byfjord tunnel, and replaced the ferry between Stavanger and the islands out towards Rennesøy. It reaches 133 metres below sea level, and the steepest gradient is 8 percent.

Rennfast made commuting to Stavanger possible for the whole island municipality, and removed the ferry from the E39 northbound. Both 1992 tunnels are now among the oldest undersea tunnels on the E39, and they are getting renewed technical equipment in these years, with periods of closure and convoy driving. Check the status before you drive.