As nobody has ever come back from Nibbana i guess we will never know exactly.
I`ve got my personal theory however that is related to science.
Especially on the idea that the energy levels in the universe never change.
Energy simply transforms into matter and matter back into energy, its the same thing in the end.
As a consequence you cannot simply disappear with no traces left.
Basically you have to transform back to the energy pool you have been initially created from into the state of the "uncreated".
An energy pool that is formless, informationless and "deathless".
In that way these buddhist terms actually make sense to me