problem with import 3DS as a ground mechs

[saveus]

now i have another pb importing 3ds as a mech ground work
but now i try to import different object and oz generate all time a flat cube

i resume the what i want to do, i try to make an inside/outside in a volcano.

in first version i use a heightmap for make the zone and make tunnel with a dongeon builder 3ds file.

but have when a try to use "split with a ground mesh".
(i suppose is due to a face too big )

i put files here if you want to see.


after this i ll try to make this directly in truespace ( i prefer it to modelling) ,
an a make a big cube and use "substract" fonction for make the zone(see sample picture) but dont work have a flat cube when import (see the picture) .




p.s. i'm working with oz only since 1 week dont know yet all functions