How do we discover what is inside the Earth?
Caves and mines go only up to 1-2 km inside the Earth. Even the deepest wells of the world, such as the one 12 km deep in the Russian Kola Peninsula or the 9-km-deep one in the Upper Palatinate, hardly scratch the Earth's crust. One can "look' deep inside the Earth in the aftermath of earthquakes or by creating nuclear explosions. The explosions generate sound or seismic waves. They also throw off rock particles. By measuring their intensity and timings, scientists discover the composition of the Earth's interiors.
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2)How did we discover that the Earth is round?
About 3000 years ago people believed that the Earth was flat. But people wondered why ships disappeared at the horizon. Sailors too returned from journeys and were proof that ships did not fall off the edge of the Earth. The simplest explanation was: the Earth must be round. The final proof came in the year 1522 after the expedition of Ferdinand Magellan, who was the first to sail around the Earth.
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