It used be one between Denmark and Germany. Likely that one you are thinking about. Canceled due to the Fehmarn Belt bridge construction.
I’m looking forwards to it as it’ll nearly halve the Copenhagen <-> Hamburg train time, down to 2 hours and 20 minutes.
> In 2025 when the tunnel (the Fehmarn Sound Tunnel) was still not approved by authorities it was revealed that it would not be opened in 2029 as it was then planned but in 2032, which would delay train traffic along the new connection until then. Road traffic can use the old bridge.
Source: https://en.wikipedia.org/wiki/Fehmarn_Sound_Tunnel
I would bet actual money that we (as in we Germans) don't get that tunnel done before 2035.