How long can you keep a secret? The Tantric Buddhists of Nepal and Tibet kept one of their arts a secret from the world until 1988, and they'll be sharing it soon with Triangle residents. From Feb. 28-March 21, two Buddhist monks in robes of gold and crimson (pictured) will create a mandala--a religious artwork made of millions of grains of colored sand--at UNC-Chapel Hill's Ackland Art Museum. Tantric Buddhists believe the ancient art of the mandala inspires feelings of peace and well-being. The two visiting monks, who speak some English, will attend a free public opening reception for Buddhist Art and Ritual from Nepal and Tibet, an exhibit of sculptures, paintings and sacred objects opening Feb. 25 at the Ackland. The mandala will be on display through June 8. Call 962-5736 for details.