And we are lucky for it. If the runaway core had reached the water table beneath, it would have exploded and irradiated most of Eastern Europe, if not the whole continent.
That's the case for any nuclear meltdown, Chernobyl isn't special in that regard. If you have a very dense blob of liquid fuel (called corium after a meltdown) generating megawatts of power it's going to melt through concrete and steel, it's just a matter of time. Once that blob hits a ton of water underneath it forcing all of that water to heat up will create enormous pressure and the only way out is up.
