Floating stones are used to build this bridge Ram Setu but now the NASA found it is under the sea however the floating stones how could sink in the ocean.
Updated: Its actually the story of war within Rama and Ravana by the way Rama and his army entering via floating bridge build by Nala. All the things are clear and understood.
Now the Floating stones why sunk in an ocean now.
The sources (mostly Puranas) which talk about floating stones don't refer to the latest Ramayana. They tell about some other Kalpa's Ramayana.
Please see Tulsidas's explanation in Ramcharitmanas 1.33 (for more details, please see this answer):
नाना भाँति राम अवतारा। रामायन सत कोटि अपारा।।
कलपभेद हरिचरित सुहाए। भाँति अनेक मुनीसन्ह गाए।।
In the latest Ramayana, the bridge was not built with stones only but trees, rocks and mountains (Yuddha Kanda - Sarga 22).
The floating stones theory doesn't seems 100% true in the current Kalpa rather it seems the great work of an Engineer/Architect.
Nala was son of Vishvakarnma (the great Architect of Devas) and got to know the art from his father. This is what Nala himself says in Valmiki Ramayana.
And a bridge made by an Engineer/Architect may collapse, at least after thousands of thousands years.
