Hezekiah's Pool (Hebrew: בריכת חזקיהו‎, Breichat Hezkiyahu) located in the Christian Quarter of the Old City of Jerusalem, was once a reservoir forming part of the city's ancient water system. As of 2010 the pool is dry and surrounded by buildings on all sides. Josephus Flavius referred to the pool as Amygdalon, meaning ‘almond tree’.  The pool is supposed to be the one referred to in 2 Kings 18:17, and there is a belief that this is the upper pool where King Hezekiah met messengers from the king of Assyria. At a later time it was fed from the Mamilla Pool, one of the three reservoirs constructed by Herod the Great during the 1st century BCE. by an underground conduit which still partially exists. The pool is also known as the Pool of Pillars, or the Pool of the Patriarch’s Bath (in Arabic: Birkat Hammam el-Batrak). 

The pool is 240 feet (73 m) by 140 feet (43 m) in size, with an estimated capacity of nearly 3,000,000 US gallons (11,000,000 l; 2,500,000 imp gal). The bottom of the pool is cemented and leveled natural rock.

http://en.wikipedia.org/wiki/Hezekiah's_Pool