Why hasn't the earth's core cooled down yet?

We have tried to tap into that energy. Incredibly, Icelandic farmers can even grow bananas (albeit in small quantities), and the reason is that they use geothermal energy from the hot Earth's interior. Iceland has a lot of hot magma near the surface, and so they use that heat to do all sorts of things. There are also other places around the world where they use this heat in the hot Earth around them to heat water and power things. But the ultimate question of where does that heat come from, is that the heat has, to a certain extent, always been there. The Earth is a huge body, and as a result, it has a huge amount of energy trapped under its surface, but it has cooled down. When the Earth was first formed, it was essentially a blob of molten material in space. Since that time it has cooled a lot, but because we're quite a big planet, we haven't lost all our heat yet. Then there's a second contribution. In the early days of the Earth when it was still molten, all the heavy and dense elements sunk deeper into the Earth's crust than the lighter ones. The heavy, dense things included radioactive elements like uranium, which continue to decay today, giving out heat as they do so. This so-called "radiogenic heat" accounts for about 90% of the planet's heat production.