The Church in the Rock (or Temppeliaukio Church) designed in 1969, remains a foremost Helsinki attraction. Hewn into solid rock in the middle of an ordinary residential square, the Lutheran church features a stunning 78 foot (24 meter) diameter roof covered in 13.6 miles (22 kilometers) of copper stripping. Natural light streams in through 180 window panes which connect the copper dome and the stone walls.
Designed by Timo and Tuomo Suomalainen in 1969, the church was originally controversial but is now a source of civic pride. The church is popular with churchgoers and secular Finns alike. It often hosts classical concerts with the raw rock interior walls providing wonderful acoustics.
It's a great tour. Specially if you don't have much time in Helsinki. :)
You can reach the church on tram 3T or 3B, or by train 14, 14B, 18, 32, 39, 39A, 39V, 45-47, or 70T.