The last major group of orchids are referred to as Terrestrial because they grow on or in the ground. However, they do not grow in the same soil that most of us understand as potting soil. They grow in humus or leaf litter accumulated on a forest floor. This humus is an extremely open and porous medium which can be duplicated with a fine bark mix in cultivation.
Examples of terrestrial orchids are cymbidiums and most of the slipper orchids.