Dark Day in the Deep Sea by Mary Pope Osborne

Jack and Annie board a 19th-century research ship and plunge into an ocean expedition filled with sea creatures, scientists, and a surprise encounter with a giant octopus. As they try to complete their mission, they learn about marine life and the spirit of exploration. When the ship faces peril, the siblings must use what they’ve learned to help save the crew. The story is filled with undersea wonder, tension, and discovery. It’s a science-based adventure with real stakes.