The intensive talks are set to be held in the days before the summit on 17th October.
The Irish Times has reported when a journalist asked a European Commission official where the tunnel was, they replied it's "not a place, but a state of mind".
However, the tunnel is perhaps best understood as Brussels phraseology for discussions held among key negotiators in complete secrecy, with no press briefings or documents published, meaning offers and counter-offers aren't disclose – unless they're leaked of course. Even the EU's 27 member state ambassadors won't be informed of any progress.
Whatever results will be a jointly-agreed proposal.