Complex sentence: Also underground, the massive cistern, known as the "Sette sale", stored much of the water used in the baths and was in fact capable of storing no less than 8 million litres, still exists today.
Split into simpler sentences: Also underground, the massive cistern, known as the "Sette sale", stored much of the water used in the baths. It was capable of storing no less than 8 million litres, and the "Sette sale" still exists today.