The Seattle waterfront is currently undergoing a multi-billion dollar overhaul and the Seattle Aquarium has for many years now considered a major expansion. The latest plans include a Tropical Pacific Pavilion and with 811,000 annual visitors last year the aquarium is already one of the 10 most visited in the United States. Here is an informative link with a few diagrams:
Seattle Aquarium aims to make splash with huge expansion - Puget Sound Business Journal
Seattle Aquarium aims to make splash with huge expansion - Puget Sound Business Journal