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The lava pool is a fairly large pocket of magma which the D'ni found on their excavation of a path to the surface. It is part of a larger volcanic fault line which spreads for some distance east to west, meandering north and south as well. The D'ni apparently avoided drilling tunnels near the fault, but still decided to construct a path that led through the pool, traversing it with long spans of specially-formulated D'ni stone able to withstand the intense heat.
Because of the earthquake damage during the Fall of D'ni, a span of the bridge across the lava pit was destroyed. It was replaced by a single beam of D'ni stone sometime later.