The thing about mysterious explosions in Siberia thought to be down to methane being released from under the tundra. Stray thought. What if there was a really big one of those in 1908? Therefore anyone thnking Tunguska was down to an asteroid impact, or a UFO's anti-matter drive malfunctioning, et c, are looking in exactly the wrong direction?
Also, Deadman's Island. It's a boat-trip away from London. Remote, uninhabited, with a local rep, and people don't go near it. Thinking like a criminal for a moment. Let's say you are involved in activities in London which you do not want drawn to the attention of the authorities, these generate an inconvenient corpse or two, and you want to dispose of the evidence. You have access to a boat, and go, perhaps, on a night-fishing trip down the Thames estuary. Where better to hide the bodies than in a cemetary island - just another corpse? are there more recent bodies there than 1815?