Rudolf Diesel's mysterious disappearance captivated the world, with media coverage and speculation abounding. Initially, his death was believed to be the result of an accident, as he had appeared optimistic about his future and colleagues confirmed that suicide was out of character for Diesel.

Soon, however, the circumstances of his disappearance were deemed suspicious. The press entertained theories that German spies or oil trusts that stood to gain from his death were responsible for his demise. Meanwhile, intimate details of Diesel's personal life, financial troubles, health concerns, and anxieties about engineering rivals surfaced in the media, seemingly pointing to death by suicide. More confusion followed: A mysterious telegram was sent to Diesel's family in Munich indicating his safe arrival in London; Dutch sailors retrieved a drowned man with Diesel's belongings, but the body was returned to the sea without a proper investigation.

As World War I erupted in 1914, Diesel's disappearance faded from public consciousness.

Brunt lays out the three foremost theories about Rudolf Diesel's disappearance: suicide, accident, or murder. Accidental death is typically dismissed—this explanation was typically a euphemistic way of describing suicide, which was seen as taboo at the time. 

Most accepted is that Diesel died by suicide. However, most of Diesel's friends and colleagues rejected this idea.