This underwater monitoring system (1) comprises: a first housing (12) of which a lower side is open to form a first opening (12a), and which has a gas (10) sealed therein and is disposed underwater; a first weight (14) which is connected to the first opening (12a) side and makes the first opening (12a) side face downwards in the water; a first float (16) which is connected to a side opposite the first opening (12a) side of the first housing (12) and causes the first housing (12) to be suspended down in the water (22); and a first optical information acquisition device (20) which is installed so as not to be in contact with water in the gas (10) inside the first housing (12) and acquires optical information about the lower side of the first opening (12a).

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