Given a text, write a compressed version of it in a single sentence.
Q: Across the tram lines from Haghia Sophia lies the entrance to one of Istanbul's more unusual historic sights - the Yerebatan Saray (Underground Palace). This amazing construction is actually an underground cistern, one of many that once stored Constantinople's water supply.
A:
Across the tram lines from Haghia Sophia lies the entrance to one of Istanbul's more unusual historic sights - the Yerebatan Sarayi.