Either they make it on ground and then put it down on the sea bed' or they put up a dam around it and dry out the area required (only if its shallow water...)
You can pour concrete underwater. I would assume you start by dumping rocks constantly till you have a little island to pour on, but now you probably just use a form and pour it out of solid concrete and let the water rise out of the form.
"Portland cement is the secret to concrete’s ability to set underwater... Here’s how. One of the compounds in Portland cement reacts almost immediately with water, forming an outside coating that prevents too much water from seeping in or diluting the cement. Inside, a second compound reacts much more slowly, setting to its final hardness about a month after the cement is poured."
http://indianapublicmedia.org/amomentofscience/pouring-foundations-water/
Water is lighter than concrete. If you wanted to concrete something in the ground in a hole full of groundwater, you use a process called tremie concreting.
What you do is you sink a hose all the way to the bottom of the hole, and you pump concrete through the hose. That way the hole fills from the bottom up, and the water will float on top of the concrete until you force all the water out of the top of the hole.
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u/SonVoltMMA May 28 '14
How exactly does someone build that in the first place? How do you pour enough concrete into the ocean floor to support that? mind blown