"Mare Tranquillitatis" is a Latin term that literally translates to "Sea of Tranquility." This term is used mainly to describe the large, flat region on the Moon's surface where the Apollo 11 mission made its historic landing. However, if you're looking for alternate synonyms for this phrase, there are plenty of options to choose from. "Calm Sea," "Tranquil Sea," "Peaceful Water," "Serenity Ocean," and "Still Sea" are all great alternatives to use. These synonyms all evoke a sense of peacefulness and stillness, which makes them perfect to use in a variety of contexts. Whether you're writing a poem, a novel, or simply looking for an appropriate descriptor, any of these synonyms is sure to fit the bill.