* If you mean the most massive objects: The two heaviest things on Earth are the Earth's core and the mantle. The core is mostly iron and nickel and makes up about 32% of Earth's mass. The mantle, composed of silicate minerals, makes up about 68% of Earth's mass.
* If you mean the heaviest things in terms of weight: This gets more complicated. We could talk about:
* The largest man-made structures: The Three Gorges Dam in China, for example, is incredibly heavy.
* The heaviest natural objects: Mount Everest, the tallest mountain on Earth, is a contender.
* The heaviest single material: It would be something extremely dense like a large chunk of osmium, the densest naturally occurring element.
So, it really depends on your definition of "heavy"!
