Not exactly conspiracy theories, but close: I've gotten interested in how actual events show up in myths and legends, and have always been interested in certain sciences, and geologic dating of various cataclysmic events has corresponded interestingly with some folklore:
The eruption (~600 years ago) of Mt. Mazama in Oregon, which created the Crater Lake caldera through a series of cataclysmic eruptions has existed, in metaphoric form, in Native American folkore from the region. There were no written languages in the area at the time, but pieces of the stories survive to present day.
The earthquake-triggered collapse of a mounatin gap in NW Turkey may have created the Black Sea - previously a frewshwater body 50% smaller. The remains of a handful of neo-lithic settlements have been found along the drowned shoreline, so there were early humans in the area, and evidence of an old river delta, now submerged, still exists on the seafloor of the Black Sea, in the form of fan-shaped sediment deposits. The theory is that this may be the source of the Noah's ark legend - Mt Ararat is located just east of the southeast end of the Black Sea.
Several episodes from the book of Exodus are also theorized to have a possible gelologic explanation: at about the same time historically, one of the largest volcanic eruptions in human history occurred on the island of Thera (now Santorini and remnant islets) in the Aegean Sea. This eruption ejected between 40 and 100 cubic km of ash into the atmosphere, detsroyed the island of Thera, and blasted a caldera crater 1500' deep into the Aegean seafloor. Thera and Crete were both part of the Minoan civilaization at the time, which was almost completely destroyed, and very, very large tsunamis moved S and E through the eastern Mediterranean. Minoan artifacts are still being found offshore from Santorini (the largest fragment of Thera that remained above sea level, and now a popular tourist destination), and debris has been recovered from topsoil nearly 100' above sea level on a few islands, signalling the extremity of the tsunamis. Egypt and the Holy land would have both been downstream from both the tsunamis and a the ashfall (and the ashfall and dust would - in an eruption of this magnitude - have had major, if temporary, effects upon the climate). It has been speculated that at least some of the phenomena described in Exodus may have been a poetic or allegorical description of the effects of the eruption; such effects would - in such an eruption - have been plausible.
The largest solidly documented volcanic eruption in human history took place at Mt Tambora in Java, 192 years ago, with an eruption that ejected at least 100 cubic km (Mt St Helens, in 1980, by contrast blasted 1-2 cubic km into the atmosphere) into the atmosphere. The climatological aftereffects included July blizzards in Canada, New England, Scandanavia, Russia and northern China, freezes in mid-summer as far south in the US as North Carolina, and a famine across eastern Europe. The famine, it is theorized, fueled the first large wave of migration from Europe to the U.S., and the summertime freezes in the NE US, it is speculated, may have provoked the first major internal migrations towards the SW and Pacific Coast of what is now the US.
Of the many disosaur-extinction theories there is this: there are 4 major impact craters on Earth, now buried under enough sediment as to be fairly obsured, though evidence would be seen in a fragmented bedrock (which re-cemented back together from the heat of the impact) core sample. The craters are about 60 miles wide each, one along the Yucatan coast in Mexico, one in the North Sea off the NE coast of the UK, one in the Arabian Sea 200 miles west of Mumbai, and one in the Ukraine between Kiev and the Byelorussian border. Each are of the same age, and correspond (65 million ya) to about the same time as a major extinction event at the Permian/Triassic boundary. Core sample dating reveals (from locations around the Earth) a thin layer of elevated iridium content, an element normally not found in such concentrations at the earth's surface, implying that the Ir may have been deposited as fallout from an ash/debris cloud. Fossil debris from buried topsoil layers in several Caribbean islands (found at elevations well above sea level) is also of roughly the same age.
The latest on the Tunguska event, over Siberia during the early 20th century, is that a team *may* have found an impact crater: a small, but very deep lake, with a sloped bottom and smashed bedrock below the deep end of the lake. This is located several miles away from the center of the airburst (the center of the area in which trees were incinterated and laid flat in a broad butterfly shape). The UFO and Black Hole theories are still in existance with this event, along with the general meteorite/comet theory. The lake findings would suggest a meteorite, though the above-ground explosion and the pressure shock waves that radiated away from the site would suggest a comet explosion in the lower atmosphere...
There are several other volcanic events that are thought to have had epic effects on human development. It's been noted that homo sapiens has surprisingly little genetic variance for a species ~100,000 years old, and one theory proposed that humans went through a genetic narrowing ~75,000 years ago, which corresponds with the eruption of Toba in Sumatra. The eruption created a crater lake (Lake Toba) 100 miles long, 30-60 miles wide and several hundred feet deep. This would probably be the largest eruption during human existance; ashfalls 20 feet thick have been found in core samples as far away as India. It is speculated that such an eruption (about 1000 times larger than the eruption of Mt St Helens) would have killed 3/4 of the human population alive at the time (about 1,000,000), would have driven numbers of surviving homo sapiens from east Africa, would have probably triggered an ice age in the Northern Hemisphere (thanks to the trade winds - much of Europe, northern Asia, and central Asia would have probably become uninhabitable) and would have created an ozone layer hole that would have exposed parts of the Southern Hemisphere to temporarily (1-5 years) highly elevated UV layers.
...A bunch of semi-scientific what-if's...