A century ago, science barely knew that the Earth even has a core. Today we are tantalized by the core and its connections with the rest of the planet. Indeed, we're at the start of a golden age of core studies... Our main tool for core research has been earthquake waves, especially those from large events like the 2004 Sumatra quake. The ringing "normal modes," which make the planet pulsate with the sort of motions you see in a large soap bubble, are useful for examining large-scale deep structure.
The inner core of the Earth, its innermost layer as detected by seismological studies, is a primarily solid sphere about 1,220 km (758 mi) in radius, only about 70% that of the Moon. It is believed to consist of an iron-nickel alloy, and it may be hotter than the Sun's surface.