The third generation of gravitational wave observatories and their science reach RID G-1113-2011 RID F-8892-2011 RID C-7233-2008 RID B-7861-2009 RID A-3864-2010 RID B-7878-2010 RID F-7100-2011 RID A-2445-2010 RID D-5236-2011

Large gravitational wave interferometric detectors, like Virgo and LIGO, demonstrated the capability to reach their design sensitivity, but to transform these machines into an effective observational instrument for gravitational wave astronomy a large improvement in sensitivity is required. Advanced detectors in the near future and third-generation observatories in more than one decade will open the possibility to perform gravitational wave astronomical observations from the Earth. An overview of the possible science reaches and the technological progress needed to realize a third-generation observatory are discussed in this paper. The status of the project Einstein Telescope (ET), a design study of a third-generation gravitational wave observatory, will be reported.