-In the subsea tunnels a bit further east, in Norway, large fans mounted at regular intervals help maintain a draft through the tunnel.

I don't think much forced ventilation is needed for normal operation - the blades spin very leisurely, if at all - but in case of fire they can direct the smoke and heat away from the bulk of the people trapped inside.

The fans look like good ol' jet engines.[0]

[0] https://www.systemair.com/fileadmin/_processed_/c/7/csm_Syst...