The Kingsroad

With an alliance created between Gracklstugh and Kraggenhammen, a large tunnel was constructed to directly connect the two cities together. Known as the Kingsroad, this tunnel is wide enough for two railway tracks and cart paths on both sides. The Kingsroad serves as the major trading route between the surface and Underdark, with trains powered by large Kinetic Engines travelling frequently between the two cities, shipping freight and cargo. It's completion resulted in a major boom in wealth and prosperity between the two cities, resulting in a major expansion of Kraggenhammen's South Pit.
Construction took 2 years to complete and required digging teams from both Kraggenhammen and Gracklstugh working from both ends. During this time, a friendly rivalry arose between the digging crews from both cities as to which one would reach the halfway point first. Due to their extensive experience digging through the hard rock of the Underdark, the team from Gracklstugh ultimately reached the midpoint first, and the two tunnels reached each other around 100 m on the Kraggenhammen side of halfway. To this day, the Kraggenhammen team claims that the Gracklstugh team only won because they started a day earlier, when in reality, the Kraggenhammen team had started a week before.
Another famous anecdote regarding its construction is that, upon joining the two halves of the tunnel together, the teams discovered that they were not perfectly aligned, and that they were instead offset by a hands width. Always seeking perfection in their craft, an additional month of digging was completed to widen the tunnel by a hands width on both sides, such as they were now perfectly aligned.
Construction was briefly delayed due to the coup instigated by Themberchaud the Wyrmsmith.