Ultra-low-frequency electromagnetic waves Pc1 (0.2–5 Hz); commonly known in the literature as «pearls» are excited in the outer radiation belt and propagate toward the Earthʼs surface along geomagnetic field lines as Alfvén waves. This discovery has had a profound impact on the development of magnetospheric physics. A series of pearls sometimes arise spontaneously; without apparent cause; and sometimes under the influence of triggers. This article; dedicated to the anniversary of the discovery; examines the triggered excitation of pearls. A classification of triggers acting on dynamic systems of the magnetosphere is presented. Two types; four classes; and eight kinds of triggers are introduced. Examples of triggers of natural and artificial origin are given. The concept of a trigger cascade is introduced. Particular attention is given to the anthropogenic periodic exogenous trigger of the pearls. It manifests itself as the so-called Big Ben effect. The essence of the effect is that a series of pearls is often excited immediately following a universal time marker. It is argued that the connection between the pearl excitation and the time markers is not random; but rather represents a rather mysterious geophysical phenomenon. It is hypothesized that the Big Ben effect occurs due to the impact of an unknown type of endogenous periodic artificial trigger on the radiation belt.