Was it the Mediterranean before it dried into small lakes? What was the biggest one before that?
The Tethys, which was previously an ocean, became landlocked by the Indian subcontinent to form a massive body of water before the Indian plate collided with the Euarasian to form the Himalayas (about 250-400 Ma).
I don't see any visuals on the Tethys as a sea (as opposed to when it was an ocean) widely disseminated on the web. Here's an approximate diagram from my memory of a textbook:
The Taklamakan. BEHOLD!
I'm speculating but it sure looks like the dried bed of an old lake or inland sea to me! It doesn't stretch the imagination to consider methods for filling it with water. Divergent weather patterns, ice-age, etc. Plus, before india began piledriving asia it was actually submerged (by the aforementioned Tethys sea) and portions are still below sea level. A likely candidate.