1808 mystery eruption
It is believed that a few years before the 1815 Mount Tambora eruption that caused the Year Without a Summer, there was another strong eruption, with the sulfate that it injected into the atmosphere being detectable in ice samples. The volcanic ash that this eruption sent up into the atmosphere meant that crop yields had already been somewhat poor for a few years in a row before the 1816 disaster. The thing that makes this a “mystery” eruption is that there are not any records of any eruptions within the right time frame and in the right location to match the evidence we are seeing.
There are records of “dry fog” in Bogotá and Lima in 1809, which is suggestive of a volcanic eruption occurring somewhere to those countries’ southwest, probably in the Southern Hemisphere, but not further than 20° south.
The best guess for the location of this eruption is that it probably occurred somewhere in the southwestern Pacific, between Indonesia and Tonga. There were no Europeans in the area at the time (except people just sailing through) and local histories of eruptions cannot be dated accurately, so this can’t be confirmed. However, it is a best guess.