The 1974 Super Outbreak was largest and most violent tornado outbreak of the 20th century, and ranks just second behind the late April outbreak of 2011. In all, there were 30 F4 and F5 tornadoes reported, of 148 that touched down in the 18 hour period. According to the National Weather Service storm report, the outbreak caused 315 fatalities, 6,142 storm-related injuries, and a damage estimate of $600,000,000.
Today marks the anniversary of the April 3-4, 1974 outbreak. I have put together a visualization showing the paths of all 148 tornadoes over these two days. The circles are sized by and styled by the strength of each tornado. You can find the interactive version on my GitHub page, or click the image below.