There's a new HBO drama about the core breach and fire at Chernobyl. The 2nd episode became available today. Here's the trailer.
YouTube will provide you with all the documentaries and information you could possibly watch but this drama makes it real.
Here is the story you need hear. There were huge water tanks under the reactor. They needed to be emptied before the core melted through and contacted them. If that had happened, the resulting explosion would have been between 3 and 5 megatons. Megatons. Remember the Hiroshima bomb was 20 kilotons**. It would have torn open the other reactors at the site and dispersed enough radiation to make the western Soviet Union and most of Europe uninhabitable.
Three men volunteered to enter the flooded basement under the burning reactor to find and open the valves. They succeeded. They took high doses of radiation and some reports suggest they died but from what I can find it appears they survived the initial period of acute radiation sickness.
Their names were Alexei Ananenko, Valeri Bespalov and Boris Baranov. They saved millions of lives and a continent.
UPDATE: In the comments, the physics and math question of the potential size of the explosion is generating some thoughtful questions. It may be like the old game of "telephone", where the actual risk was of the steam explosion ejecting the core material into the atmosphere along with all the radioactive water, resulting in a down wind radiation fallout equivalent to a 3-5 megaton bomb. If it had done that as well at causing the other reactors to fail, the outcome would would have been bad enough. I am genuinely curious now and want to understand. If I find a definitive answer, I'll post it.
**Edited to correct yield per Old 1811