Great Mangrove Barrier

The Great Mangrove Barrier divides the Koan homeworld of Sera across its prime meridian. It stretches for miles and is too thick in most places for modern ships to cross. Nor would many want to, as it divides the planet between its civilized waters and the dangerous sea of leviathans.
Only the Koan vandal tribes inhabit the barrier, hunting leviathans and keeping watch for large-scale crossings. Occasionally, disturbances in leviathan ecosystems will cause sudden migrations into civilized waters called leviathan stampedes. So, any civilization hoping to take root on Sera is wise to hold some presence in the barrier's dangerous climes.
For the Koan Republic, that presence is largely handled by the Leviathan Hunter's Guild, a private organization that maintains a gate on each side of the barrier. These gates are the only means of ingress and egress through the barrier, and the guild only allows its highest-ranking members through.