There have been many ideas put forward to explain why the cave was dug. It is very unlikely that it was the castle dungeon. The quality of workmanship also rules out the idea that it was just the castle cellar, or a sand mine. The through passage could have been dug as a sally port, which is an escape tunnel to allow the besieged inhabitants to surprise their attackers, or to escape unnoticed. This does not explain why the large side passage was dug.
The effort and skill used to dig The Barons’ Cave, and the size of its galleries, suggest that it was a special and important feature of the castle. The story which gave rise to the name "Barons’ Cave" is that the barons met there to draw up the Magna Carta in 1215, before making King John sign it. Unfortunately, this is a romantic story that is certainly not true.
Nobody knows how exactly old the Barons' Cave is. The oldest reference to it dates from 1586 when Camden describes "an extraordinary passage with a vaulted roof hewn with great labour out of the soft stone." Doors and windows with a similar profile to the cave passages were being built from about 1200 onwards, but we must be careful before drawing any conclusions from this.
In the centuries that followed the cave was almost certainly accessed for sand with the rough side passage probably dug in the 19th century by sand diggers, who were also active elsewhere in Reigate at that time. Throughout the cave, the work of sand diggers can be seen. Many alcoves have been dug into the otherwise well-shaped walls. The walls of the cave have attracted graffiti artists. Apart from the many initials, names and dates from 1644 onwards, a number of other carvings have been made. The most notable of these are a series of large heads, each one unique, and possibly meant to depict real people. There are also a number of horses' heads and a bull's head to be found in the cave.
The cave has a long history as a local curiosity. The earliest account of guided tours found so far dates from 1860, when a lady from a nearby cottage had to be summoned to conduct curious visitors around the cave. Visits continued until the 1970s, with the Castle Grounds' gardener acting as guide. In 1991, the Wealden Cave and Mine Society reopened the caves for the public after a period of restoration.