It is thought that Stonehenge was built in three phases from 2950-1500 BC. An outer ditch with an inner bank of rubble and a double ring of 56 postholes, with the Heel Stone at the northeast, date from the first phase. A double ring of bluestones was set up during the second phase, but the stones were later moved to the centre and laid in a horseshoe formation, while tall trilithons were erected in a circle around them. Still today, the significance of the structure continues to intrigue and raise questions.