| Summary: | Plain Language Summary Some ice caps and mountain glaciers contain water bodies that drain episodically and catastrophically as subglacial floods, sometimes threatening human life and property. Our observations suggest that we unintentionally triggered such a flood from a subglacial lake in Iceland by boring through the overlying ice. Our conduit allowed water inside the glacier to drain into the lake, causing the pressure to rise and lifting the ice shelf enough to allow the lake to rapidly drain beneath the glacier toward its edge. This event revealed a mechanism by which natural drainage events can trigger such floods and why floods from this particular lake are more likely to occur in summer. Glacial floods (jökulhlaups) are a phenomenon of some temperate ice masses; they are a significant natural hazard, but their complex hydrology is incompletely understood.We document a jökulhlaup from a subglacial lake in Iceland that was inadvertently triggered by a borehole drilled through the overlying ice.We propose that this borehole allowed an englacial water body to drain into the lake, inducing a transient rise in pressure that overwhelmed the lake's subglacial seal 5 days later. Runaway melting of a subglacial conduit by 4◦C lake water then initiated a flood to the outlet glacier margin. This incident suggests that draining of englacial water bodies via hydrofracturing crevasses and flooding of moulins by precipitation events are potential natural triggers of jökulhlaups and explains a correlation between surface melting and jökulhlaups. This hydraulic trigger could have wider implications for relations between meteorological conditions, drainage, and dynamics of some glaciers.
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