From Haghia Sophia to Beyazit Square - Strolling Through Istanbul: The Classic Guide to the City - page 135

Restoration of the cistern began in 1985 and it opened to the public
in 1988.
The structure was known in Byzantium as the Basilica Cistern
because it lay underneath the Stoa Basilica, the second of the two
great squares on the First Hill. The Basilica Cistern was built by
Justinian after the Nika Revolt in 532, possibly as an enlargement of
an earlier cistern of Constantine. Throughout the Byzantine period
the Basilica Cistern was used to store water for the Great Palace
and the other buildings on the First Hill, and after the Conquest
its waters were used for the gardens of Topkapı Sarayı. Nevertheless,
general knowledge of the cistern's existence seems to have been lost
in the century after the Conquest, and it was not rediscovered until
1546. In that year Petrus Gyllius, while engaged in his study of the
surviving Byzantine antiquities in the city, learned that the people
in this neighbourhood obtained water by lowering buckets through
holes in their basement floors; some even caught fish from there.
Gyllius made a thorough search through the neighbourhood and
finally found a house through whose basement he could go down
into the cistern, probably at the spot where the modern entrance is
located. As Gyllius writes, referring to the Stoa Balica as the Imperial
Portico:
The Imperial Portico is not to be seen, though the Cistern
remains. Through the inhabitants' carelessness and contempt
for everything that is curious it was never discovered except by
me, who was a stranger among them, after a long and diligent
search for it. The whole area was built over, which made it less
suspected that there was a cistern there. The people had not the
least suspicion of it, though they daily drew their water out of
the wells that were sunk into it. By chance I went into a house
where there was a way down to it and went aboard a little skif.
I discovered it after the master of the house lit some torches
and rowed me here and there through the pillars, which lay
very deep in water. He was very intent on catching his fish,
with which the Cistern abounds, and speared some of them by
the light of the torches. There is also a small light that descends