Indoor vs Outdoor

In the natural world caves are found outdoors, under the ground. But that doesn't mean we have to be so restricted in our vision. Entre-Prises caves can be buried underground, sit on the surface or float in the air! Or alternatively they can be built inside a building. All caves, however are not equal and some considerations have to be made:

Indoor
Indoor caves are usually made from polyester resin and glass fibre as spraying concrete indoors is often not very practicable. Indoor caves provide the experience of caving without some of the discomforts of natural or outdoor caves: the air temperature can be relatively easily controlled, in most cases specialist clothing is not required The passage walls are not very thick so more cave passage can be designed to go into a small area
   
Outdoor
If an outdoor cave is to be buried within a hillside it must be able to withstand the bearing pressure of any overlying material eg soil or snow. This is usually addressed by making the caving structures in sprayed concrete that has been specifically engineered to withstand the loads. Alternatively the cave passages can be constructed in polyester resin and glass fibre but then they have to be within a structural 'box' if soil pressure is to be applied.