Old movie where bristish explorers where against an indian sect hiding underground but they found the access to the underground refuge it was a bark door in a big tree in the forest (jungle). Explorers where using boats on a small river just before finding the access and they were wearing pith (safari) hats . Ounce inside the access they had to face collapsing floors, fire. Indiana Jones style. I saw that movie at the theatre in the early sixties in north america.