The below-sea worlds of Finding Nemo and The Little Mermaid almost make us wish we had gills, but now you don't have to hold your breath to experience what it's like to hang out beneath the waves. Architects all over the world are embracing the idea of building underwater hangouts in the ocean. Such projects are difficult to pull off because of safety concerns, engineering challenges, and extremely expensive building costs, but that hasn't stopped the most daring designers. Check out our list of 20 Unbelievable Underwater Destinations.

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