In order to gain further understanding on the mechanism of rapid seam depth variations, the so-called spiking, in deep penetration laser beam welding, this effect is analyzed and explained using the example of laser welding of ice. Laser welding of ice provides the advantage that the temporal behavior of the vapor capillary can be analyzed by means of common imaging methods in the visible spectral range. The occurrence of spiking and its frequency is found to depend on the feed rate but is not inﬂuenced by instabilities of the rear wall of the capillary. Also, the effect of downward moving structures at the capillary front can be shown to be independent from root spiking. Spiking is found to be governed by a periodically alternating irradiation of the capillary front wall by the incident laser beam in the lower part of the capillary. This is veriﬁed by ray-tracing calculations on capillary geometries extracted from high-speed videos. These ﬁndings are further substantiated by a transient smoothed particle hydrodynamics model to simulate the melt ﬂow and evaporation at the capillary front that was coupled to the ray-tracing calculation to determine the local absorption of the laser radiation within the capillary.