DART Underground (Irish: DART Faoi Thalamh), also known as the Interconnector or DART+ Tunnel, is a proposed mainline-rail tunnel in Dublin, Ireland. First proposed in 1972, as of 2021 it was not funded or scheduled. While the Greater Dublin Transport Strategy 2016-2035 (published in 2016) included the DART Underground as a proposed National Transport Authority project, the tunnel was not included in the National Development Plan published in 2018 or DART+ expansion plans published in August 2020.