Ancient Celts believed a parallel realm called the Otherworld was accessible through sacred 'thin places' in nature.

The Otherworld, known as 'An Saol Eile' in Irish, is a fundamental element of Celtic mythology preserved in medieval manuscripts such as the 12th-century 'Lebor Gabála Érenn' (The Book of Invasions). These texts describe the Tuatha Dé Danann, a supernatural race that withdrew into the hills and mounds of Ireland following their defeat by the Milesians. These mounds, called 'sidhe,' were understood as physical gateways to a realm where aging and hunger held no power.Many of these 'thin places' align with Neolithic passage tombs, including Newgrange in County Meath, constructed around 3200 BCE. These monuments were built with remarkable astronomical precision, oriented to capture the winter solstice sunrise—a phenomenon the Celts likely interpreted as a moment of cosmic alignment and spiritual significance. The Otherworld concept helped early Celtic societies make sense of time's unpredictable nature and environmental mysteries.Scholars including Nora Chadwick and Barry Cunliffe have documented that the Celtic calendar was structured around two seasons: a light half and a dark half. Samhain, celebrated around November 1st, marked the transition into the dark half and was considered the threshold when natural laws temporarily suspended, creating ideal conditions for encounters with the supernatural. This worldview provided a spiritual framework for understanding seasonal cycles and the profound mystery of death.