The discipline that deals with the study of ancient undergrounds is called “archeology of artificial cavities”. In the past, the definitions “underground archeology” and “urban speleology” have also been used, but they have fallen into disuse because they are inaccurate. In fact, the first is overly generic since archeology deals almost always with buried objects, even in the absence of empty underground spaces. The second one does not hit the mark because speleology is not a field of research but only the set of techniques used to access and move in underground environments. Also, not all artificial underground are in urban areas: aqueducts, for example, often develop for miles in rural or mountainous areas.
Like all scientific disciplines whose purpose is to obtain reliable and usable data for further research, even in archeology of artificial cavities, it is essential to use a proper and appropriate method. This method is basically that of modern archeology, based on identifying and linking stratigraphic units, recognized as the product of individual events, on the production of objective documentation, on the classification of artefacts in typologies and on the search for comparisons. The techniques used, on the other hand, are in part different from those of a normal stratigraphic excavation or surface survey, since they have to adapt to the often difficult subsoil reality.
About the progression, techniques developped to move into natural cavities are used, which in the whole fall under the name “speleology”. Rope progression is used to pass vertical slopes, using descenders and clamps. Devices such as helmets, bodysuits in synthetic cloth, knee pads, work gloves, boots, are used for body protection. Neoprene wetsuit are needed when you are in flooded places. For the lighting mainly headlamps with LEDs fixed on the helmet are used.
There are many knowledge that are needed in different fields. In addition to good archaeological education and knowledge of speleological techniques, it is essential to have good physical training to move easily into wells and stretches. You must then be familiar with many essential security and first aid practices to prevent difficulties and accidents or to limit damage. In order to produce the documentation, you must also master the topographic techniques, both with total station theodolite and compass, and those of photography in poorly lit places. Also, archival studies and palaeography notions are required to find the information contained in the ancient documents, often crucial for identifying hypogeal function and chronology.
Finally, the most important requirement is curiosity. Often undergrounds are confused for adventure paths where they go to show their courage or to satisfy their adrenaline thirsts, but in fact, the only reasonable motive to go underground is the desire to learn new things. Of course it is not forbidden to have fun (indeed it is desirable!), but always respecting the places and having it clear that you are in an archaeological site and not in a amusement park.