The Overwhelmingly Large Telescope (OWL) is a concept design by the European Southern Observatory (ESO) to create a $1.2 billion USD telescope with a 100 meter (328 ft) lens, which would the largest optical telescope ever built. The scale has since been decreased to a 60 meter (200 ft) lens. The target date for completion is around 2019, and the plains east of Antofagasta, Chile are being considered for the site, due to its year-round clear skies and high altitude.
If completed, OWL would be able to observe objects 1000 times fainter than the Hubble Space Telescope. Although some interferometry projects will have a greater angular resolution than OWL, it will have greater light-gathering and imaging capacity, allowing for sharper pictures of faint and distant astronomical objects.
Because there is no technology in existence to make a 60 meter lens, any lens for this purpose would have to be segmented. To get an idea of why it is called the Overwhelmingly Large Telescope, imagine a mirror with a diameter the size of a 20-story building. Due to its huge size, cost, and complexity, it is no surprise that a feasibility review board for the ESO decided that OWL should be put on the back burner of their priority list.
Although OWL may still see the light of day, for the most part the ESO is currently focusing on the European Extremely Large Telescope, a scaled-down telescope with a 40 meter lens, for a cost of approximately $800 million US Dollars (USD). This would still be precise enough to resolve the atmospheres of extrasolar planets, which would let us do spectroscopic analysis to determine the possible presence of organic compounds.
Using a technique called adaptive optics, which was developed to reduce atmospheric twinkling, OWL or something like it will surpass the space observatories as the world's greatest windows to space. Because it is so expensive to launch payloads into space, and comparatively cheaper to build a very large telescope on Earth, it is likely that the latter will dominate astronomy until launch costs can be brought way down.