nyc tunnel plug The tunnel plug in a New York City subway. MTA/Flickr

As climate change escalates, cities around the world become increasingly at risk for flooding. By 2100, sea levels are expected to rise three to four feet — and are now rising faster than they have in 28 centuries, according to a 2016 study. Sea level rise could displace 13 million Americans within the next century.

New York City already knows the potential risk first-hand. In 2012, superstorm Sandy ripped through the city and flooded its subway system.

To prevent future flooding, the city hopes that giant inflatable "plugs," made of interwoven Kevlar belts and waterproof fabric, will save its tunnels.

The plugs, developed by the US Department of Homeland Security and engineering firm ILC Dover, work like balloons that keep water from entering the tunnels. If the city knows that another serious hurricane is coming, officials can shut down at-risk stations and inflate the plugs (which can also stop gas and smoke in the event of a terrorist attack).

NYC's Metropolitan Transportation Authority started testing the plugs in 2013. And according to Popular Science, ILC Dover test and installed an improved prototype in an American city this year, though it declined to say where due to security reasons.

nyc tunnel plug NYC Governor Andrew Cuomo and a tunnel plug on the first anniversary of Hurricane Sandy. MTA/Flickr

Solutions like the tunnel plug could be crucial. In 2016, four years after Sandy , the city is still repairing the damage in many of its stations, which have  an estimated 5,600 openings vulnerable to floods . In 2019, the L Train, which goes from Brooklyn to Manhattan, will shut down for 18 months for re-construction.

But giant plugs are just one possible option for flood prevention in NYC. The subway system already uses 700 pumps that drain an average of over 13.2 million gallons of water (nearly 20 Olympic-sized swimming pools) a day from its stations. The city is also experimenting with the "Flex-Gate," large waterproof sheets that cover subway entrances to keep water out.

Cities around the world are exploring other ways to mitigate flood risks. Shanghai, a city that's vulnerable to sea level rise, will install over 125 miles of walls along nearby rivers by 2018. London already has a similar river gate, called the Thames Barrier, that spans 1,700 feet along the Thames River and protects about 50 square miles of central London from tidal surges.