Pumps basically. If the New York subway for example wasn’t constantly maintained with water pumps it would indeed fill up with water.
To quote [this](https://www.nysun.com/new-york/inside-the-mtas-fight-against-subway-flooding/35672/) article.
>On a dry and sunny day [the head of the NYC Transit Authority’s] department’s 700 or so pumps, at about 280 locations, push 13 million gallons of water out of the subway system and into New York City’s sewers. That’s the equivalent of all the wastewater produced by the city of Boca Raton, Fla., every day.
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