Financed by the Venezuelan government, the ALBA-1 cable under the Caribbean was laid with much fanfare in 2010 and connected to Cuba in February 2011. Since then it has been dormant and the government silent as to why.

"I had my eye on it. People have been asking why has this cable not been in use," said Doug Madory, who works for global Internet monitor Renesys and first reported cable activity on Sunday on the U.S.-based company's web page.

When fully operational, the cable will provide download speeds 3,000 times faster than Cuba's current Internet and be capable of handling millions of phone calls simultaneously, the government said when it was being laid.