Complex sentence: Collectively, they contain 31,000 km 3 (7400 cu mi) of water, which is more than either Lake Baikal or the North American Great Lakes, and constitutes about 25% of the planet's unfrozen surface fresh water.
Split into simpler sentences: Collectively, they contain 31,000 km 3 (7400 cu mi) of water, which is more than either Lake Baikal or the North American Great Lakes. This total constitutes about 25% of the planet's unfrozen surface fresh water.