The centerpiece of north Sumatra , lake toba is bracing climate and magnificent panoramas . clear the mind and soothe the soul . for decades a magnet from regional and foreign visitor alike , toba has developed into a full - feature highland resort while retaining the rustic charm and relax ambiance that define toba is attraction . form by a stupendous prehistoric volcanic explosion , the 100 km long lake is the largest in southeast Asia and one of the deepest and the highest in the world . the drama of that cataclysmic birth persist in 500 meter cliff drop into the blue-green water , surround by steep , pine cover slop , the climate is fresh and pleasant , with just enough rain to support the lush vegetation .