[davrosthedalek]

This is in preparation for starting construction work on the Electron-Ion-Collider (EIC) which will use the same tunnel and experiment locations.

[gnufx]

As I recall, RHIC itself replaced some cancelled project. I remember the tunnel being at least partly there in the mid-80s, with a plan to trundle ions from the tandem lab through a crazy long beamline across the site and stop nuclear structure research there as a result.

[phongn]

ISABELLE, which was a cancelled proton-proton collider. Major delays with its magnet design meant that it was overtaken by existing programs at CERN and Fermilab. RHIC reused its hall.