[Wachearex]

The core apparently was Molten liquid, they reckoned that if the core hadn't cooled it could have literally burned down through the earth's crust and carried on going down until it cooled off.

The entire core was well short of being liquid given the melting points of graphite and uranium oxide are in excess of 2000 K, but they were parts of it that had become molten. The real reason why they dumped so many materials on to it was to extinguish the different sources of ignition (such as the oxidation of graphite, the reaction between steam and the fuel rods and the chain reaction of the fuel) and to prevent the core from reaching deeper into the building sub-structure.

if it was a simple case of menting it's way though the ground, I expect they'd've welcomed it.

Actually they worried about that happening because it could create further explosions if it came into contact with the water table and it would contaminate further areas of land. Ideally they wanted it contained with the reactor building so that they could (eventually) encase of the whole structure.

As you can imagine the helicopter pilots unknowingly were subject to masses amounts of radiation and died in the forthcoming weeks. Hundreds of people at the accident were exposed to far higher doses than the helicopter pilots but the total death toll after several weeks was around 30 - none of which were the pilots: they were actually safer than the ground workers once they stopped direct dumping. Of the people who died in later years, it's not possible to say with absolute certainty that the severe acute dose they received was responsible for their deaths.