Why was this done in Kikar ha'Yarden?
Refer to Melachim I, 7:46:1:1.
What is "b'Ma'avei ha'Adamah"?
'Rashi' citing the Targum: B'Ovei Gargishta (in the thick earth). Radak ? this is good earth for furnaces in which to make molten Kelim. However, such big Kelim cannot be made in furnaces! Rather, he poured the molten copper into molds in the earth.
Here it says that he made them between Sukos and Tzeredasah. In Melachim I, 7:46 it says Tzarsan?
'Rashi' citing his uncle: Tzeredasah was Yaravam's place. Yaravam ben Nevat was an Efrasi from Tzeredah. [Yaravam's] mother was Tzeru'ah, a widow (Melachim I, 11:26), because a Mes has Tum'as Ohel, like a Metzora, and the same applies to idolatry (Avodah Zarah 48b). It says "widow", for [Yaravam] 'widowed' Yisrael from (severed their connection to) their G-d. It seems that Min Tzeredasah hints to Hetzar (he constricted) Torah, and did not widen it. He constricted the creed of Elokeinu.
Radak: Tzeredasah is close to Tzarsan (we need not expound the change).
Malbim: The name changed to Tzeredasah before Ezra wrote Divrei ha'Yamim.
