BitTorrent (BT) plays an important role in Internet content distribution. Because public BTs suffer from the free-rider problem, Darknets are becoming increasingly popular, which use Sharing Ratio Enforcement to increase their efficiency. We crawled and traced 17 Darknets from September 2009 to February 2011, and obtained datasets about over 5 million torrents. We conducted a broad range of measurements, including traffic, sites, torrents, and users activities. We found that some of the features of Darknets are noticeably different from public BTs. The results of our study reflect both macroscopic and microscopic aspects of the overall ecosystem of BitTorrent Darknets.