The Seaway is a combined viaduct/aqueduct which serves as a boat-traversable access point to the superfortress, bypassing the rocky and navigational waters around the island's south shores.

The Seaway is accessible via locks in between the two Gateway Islands. It traverses north to the main shore, as well as branches east into the “porthole”.

The Seaway and access locks are constructed from the remains of the Tricourse's south span, specifically pillars C5-C8.