From Wikipedia

The Vanished Capital Of The Pharaoh

In the Nineteenth Dynasty of Egypt, Avaris regained its past glory when Pharaoh Ramesses II founded his new capital at this old site. The city was now called Pi-Ramesses Aa-nakhtu, meaning "House of Ramesses II, Great in Victory", though it previously served as a summer palace under Seti I.

The decision by Ramesses II to transfer his government and official residence this far north from Thebes may have been influenced by his family connections, being born and raised in the area but geopolitical reasons may have been of greater importance. The troublesome Egyptian vassal states in Asia, lay much closer as did the border with the hostile Hittite empire. Intelligence and diplomats would reach the Pharaoh much quicker. The main corps of the army were also encamped in the city and could quickly be mobilized to deal with incursions of Hittites or Shasu nomads from across the Jordan.

The site of Pi-Ramesses, recorded as being located on the then eastern most branch of the Nile, was lost for more than 3,000 years and was long considered the "Holy Grail" of Egyptology. The ruins at Tanis were discovered in the 1930s by Pierre Montet and the buildings and monuments of Ramesses found led early archaeologists to erroneously identify Tanis as the site of Pi-Ramesses, based on the "masses of broken Ramesside stonework [which] were visible in the ruins of San el-Hagar (ie. Tanis). In the 1960s, Egyptologist Manfred Bietak traced all the former branches of the Nile and dated them by the pottery found on their former banks. When it was found that the Tanitic branch of the Nile (Tanis' location) did not exist during Ramesses reign while the Pelusiac branch was at that time the eastern most branch, excavations began at the site of the highest Rameside pottery location, Tell el-Dab´a and Qantir. Although there was no trace of any previous habitation visible on the surface, discoveries soon identified the site as both the Hyksos capital Avaris and the Ramesside capital Pi-Ramesses.

Built on the banks of the Pelusiac branch of the nile and with a population of over 300,000, Pi-Ramesses flourished for more than a century after Ramesses death and poems were written over its splendour. The layout of the city, as shown by Ground-penetrating radar, consisted of a huge central temple, a large precinct of mansions bordering the river in the west set in a rigid grid pattern of streets and a disorderly collection of houses and workshops in the east. The palace of Ramesses is believed to lie beneath the modern village of Quantir. An Austrian team of archaeologists, headed by Manfred Bietak, who discovered the site, found evidence of many canals and lakes and have described the city as the Venice of Egypt. A surprising discovery in the excavated stables were small cisterns located adjacent to each of the estimated 460 horse tether points. Using mules, which are the same size as the horses of Ramesses day, it was found a double tethered horse would naturally use the cistern as a toilet leaving the stable floor clean and dry. According to the latest estimates Pi-Ramesses was spread over a vast area of about 18 km² (6.9 sq mi) or around 6 km (4 mi) long by 3 km (2 mi) wide, . This made it one of the largest cities of ancient Egypt.

It was originally thought the demise of Egyptian authority abroad during the Twentieth Dynasty of Egypt made the city less significant leading to it being abandoned as a royal residence. It is now known that the Pelusiac branch of the nile began silting up c. 1060 BC leaving the city without water when the river eventually reestablished a new course to the west now called the Tanitic branch.  The Twenty-first Dynasty of Egypt moved the city to the new branch establishing Djanet (Tanis) on its banks, 100 km (62 mi) to the north-west of Pi-Ramesses as the new capital of Lower Egypt. The Pharaohs of the Twenty-first Dynasty transported all the old Ramesside temples, obelisks, stelas, statues and sphinxes from Pi-Ramesses to the new site. The obelisks and statues, the largest weighing over 200 tons, were transported in one piece while major buildings were dismantled into sections and reassembled at Tanis. Stone from the less important buildings was reused and recycled for the creation of new temples and buildings.