The Caretaker's remains in a hexi-prismatic field.
The hexi-prismatic field is an energy field that reacts directionally to sporocystian activity, like a compass.
In 2372, USS Voyager crew members placed the remains of the Caretaker into a hexi-prismatic field, so that if the remains responded to the sporocystian energy of another Nacene, they could locate the source. (VOY: "Cold Fire")