Sentence:  Caves have been, in many cultures, crucial <WSD>liminal</WSD> spaces where shamanistic ekstases occurred, bringing humans into contact with the spirits or the beyond. ... It is now a well-accepted hypothesis that cave paintings, such as the famous ones at Lascaux, must be interpreted as being part of ritual passages and actual <WSD>liminal</WSD> experiences. 
Meaning of the word marked with <WSD> tags:  Of or pertaining to an entrance or threshold. (from late 19th c.)