To take on the deepest wrecks in Scapa Flow, Ross needs to successfully navigate decompression training before heading out with local dive guide Emily Turton to dive on one of the biggest of the Kaiser's Imperial war ships from the First World War, SMS Karlsruhe. As he explores the eerie relic, Ross tries to understand how and why the German fleet ended up on the Scottish seabed, when they had already surrendered months earlier. Having resolved one enduring mystery, he then heads 600 miles south to meet with ex-serviceman Mark Culwick and dive on an experimental British aircraft carrier submarine built in the 1920s called the M2. This unique vessel was lost in shocking circumstances, with all hands. During his dive Ross starts to understand the horrifying truth of exactly how and why the 60 men on board never came home.