Sorry 3142, bad wording on my part I think!
Assuming a canyon existed down to say 3000' BELOW MSL, and I were to fly in it (ie. still flying in air, but in air of density GREATER than that of MSL, rather than less as is the usual case for flight AMSL), what would the TAS do relative to a constant CAS were I to descend from sea level down to -3000ft, in air.
Cheers, ED