After John "Fighting Branch" Loftin discovered the tunnels to D'ni in 1987. John and his friends Elias "Eli" Zandi and Dr. Richard Watson made the trip to D'ni. It was due to this interaction that Jeff Zandi became aware of the D'ni cavern. Over the years he accompanied his father to D'ni on multiple accounts.
It was during such a trip in 1995 that Jeff and his father had a substantial philosophical disagreement regarding the further of the D'ni cavern. Elias held the view that the restoration of D'ni should be physical, whereas Jeff felt strongly that the restoration should be spiritual. The result of this argument was that neither Zandi would speak to each other again.
A year later in 1996 Jeff's father died of a heart attack. Whilst he left Jeff no money (instead leaving his wealth to the D.R.F) he did leave the land surrounding The Cleft to his son.
It's unknown when or how Zandi became aware of Yeesha and her journey but he was certainly knowledgable about her plans.
Preafter
In 2001 Jeff began his plan to bring people down to D'ni, circumventing the DRC's attempts to limit footfall in the cavern.