More than a year after transforming a vacant downtown storefront into an underwater world of colorful sea creatures, marine science and imagination, Mike Bennett’s Portland Aquarium is preparing to close its doors. When the Portland Aquarium opened, Bennett made it clear it wasn’t intended to replace a traditional aquarium. Growing up a few hours from Baltimore’s National Aquarium, he visited nearly every year for his birthday. While the Portland Aquarium is coming to an end, Bennett’s work is not. Until then, visitors have one final opportunity to explore the Portland Aquarium before it closes at the end of regular business hours on Oct. 4.