___ Sea, body of water nowadays divided into "North" and "South"

ARAL

Once a vast lake in Central Asia, its shrinking waters now straddle two modern-day namesakes.

This clue refers to the Aral Sea, a once-massive lake in Central Asia that has shrunk dramatically over time. The blank signals a four-letter name, and "divided into 'North' and 'South'" hints at its current split into two smaller bodies of water due to environmental changes. "Aral" fits perfectly as the answer, matching both the geography and the letter count.