What bridge breaks when Surtr crosses it?

Bifröst breaks when Surtr’s forces cross it at Ragnarök.

Bifröst is the burning, rainbow-like bridge connecting Midgard, the human world, with Asgard, the gods’ realm. In Norse myth it is not merely a colorful natural phenomenon: it is a divinely constructed route used by the gods, with Heimdall stationed nearby at Himinbjörg to guard it.

When the sons of Muspell ride toward the final battle, their passage causes Bifröst to break. Surtr is the foremost fire giant among them and leads the assault, so later retellings often summarize the event by saying that Surtr breaks the bridge. The destruction signals that even the gods’ strongest works cannot withstand the forces of Ragnarök.

Gjallarbrú is a different bridge, associated with the road to the underworld and crossed by the dead or by travelers seeking Hel. Yggdrasil is the cosmic tree, not a bridge, while Mjölnir is Thor’s hammer. Another subtle detail is that Thor traditionally wades through rivers rather than riding across Bifröst with the other gods.