Slightly Off Topic.
The construction of Stonehenge is not only very impressive on an engineering level as others have stated, it also hold many valuable and highly enlightened mathematical, geometrical and astrological characteristics.
Here are some interesting characteristics that prove the architects knowledge of mathematics, time keeping and geometric importance:
1) The inner circle consists of 30 smaller bluestones, the last of which is half the size of the rest. This represents the timing of the Moon's phases, which lasts 29.5 days.
2) "Pythagorean Triangles" are a geometric phenomenon that consist of one Right-Angle and each side consisting of Whole Numbers. The inner circle defines the width of a rectangle inside stone henge that ends at the outside ring, when halved diagonally, a Pythagorean triangle is defined.
Furthermore, Lundy Island lies exactly Westward of stone henge. If we call the measurement between them 12 and go north a value of 5, we land directly on the "Blue Stone Site" in Wales where the 30 Bluestones where taken from, the length between the site and stone henge is 13 (relative to the other two values) which defines a Pythagorean triangle or 5, 12, 13.
3) the circle consists of Three stone circle "layers" or "walls", the difference in ratio between the inner circle and middle circle is an "octave" and is the difference between the inner and other definitions of a hexagram.
4) If the middle circle is the size of the Earth, the outer circle defines ratio difference of the centre of the moon if the moon was to contact or rest on the earth. This shows the builders knew the size of the Earth and the ration between Earth and Moon. This principle also applies to the Pyramid of Giza and most religious constructions around the globe such as the Taj Mahal.
Also, An interesting fact is that this ratio is the difference between a circle and a square of the same area, a term called "squaring the circle" which the moon to the earth, a double rainbow and the human anatomy does, as shown by DaVincii (spelling?) although the builders didn't necessarily know this, they may have.
5) The sarsen circle, held together by tongue and groove joints, show a knowledge of Pi, if the width of one stone is 1, the curved length of the stone piece (which is a fraction of the circle's circumference) is 1 x Pi, so an imaginary circle fitted inside the width has a circumference of 1xPi also. The Unit we're naming "1", the width, is 3.4757486 ft. which precisely defines a measurement known as a Royal Foot, which the Egyptians developed, suggesting trade, communication and shared knowledge between Egyptians and the builders of the Stone Circle in a England.
This measurement is also a six-millionth part of the Earth's polar radius.
The idea of a sacred place is to "marry" the Heaven to the Earth through "God's language", Which is Geometry. These methods ares till used, example: Winchester Cathedral lies at 72 degrees from North, defining 1/5 of a full turn and therefore defining a Pentagram, the shape of Life and Man between the Square (Earth) and the Hexagon (Heaven).