of Sciences in May 2006 and by the Ministry of Science and Technology of China in January 2007. It passed the CD-2 review as required by the US Department of Energy in January 2008.

Ground breaking of the experiment took place in October 2007. Detectors were assembled onsite in parallel to the civil construction. After 4 years of construction, the first of the three underground experimental halls, EH1, started data taking on August 11, 2011. Data was used to study the detector performance, resulting a paper submitted on 28 February 2012 [@ad12]. This publication reported that the relative detection uncertainty of two ADs was only 0.2%, much better than the designed value of 0.38% documented in the CDR.

Since the detector fabrication was out of sync with the civil construction, the collaboration decided to operate the experiment with two phases to maximize the scientific reach. The first phase was run with 6 ADs out of a total of 8, with two in EH1, one in EH2, and three in EH3. The second near hall, EH2, was ready on 5 November 2011, and EH3 started data taking on 24 December 2011. On 8 March 2012, the Daya Bay collaboration announced the 