THE DIMENSIONS OF THE SPHERE solid Earth an almost perfect sphere, with a radius the kilometres. you could somehow dig downwards absolute centre of the world, you'd have to dig a hole kilometres deep-directly downwards. To put this ta perspective, if you are metres tall, you'd have to dig a hole more than four million times your own height The Earth is the densest planet in the Solar system, with average density of 5.5 grams per cubic centimetre. This is about five times that of water, and five thousand times that of the air we breathe. PRIMARY AND SECONDARYWANES The force from an earthquake travels through the centre of the planet as energy waves, these can be classified as primary and secondary waves. The primary, or Rwaves are direct and lengthwise, whereas the secondary, or S, waves are sideways. These waves have helped us determine the Earth's structure. Secondary waves travel slower through the Earth than primary waves, Secondary waves also cannot travel through liquids. Geologists can use seismic meters, or seismographs, to monitor tremors in the Earth Following an earthquake, the dinerence in arrival times at the sensors can be used to shed light on the structure of the Earth's interi As the sophistication of selsmic sensors has ncreased, events besides earthquakes are now detectable: nuclear explosions, undersea landslides, avalanches, and unknown tremblings deep within the Earth