Image of the 4th century Boudhanath, Kathmandu, Nepal. This one of the holiest Buddhist sites in Kathmandu. The stupa's massive mandala makes it one of the largest spherical stupas in Nepal and the ancient Stupa is one of the largest in the world. As of 1979, Boudhanath is a UNESCO World Heritage Site. The Stupa is said to entomb the remains of Kassapa Buddha.