An article published by Jamestown Foundation, a U.S. Think Tank, gives an excellent overview of China's ambitions to link Taiwan with China via tunnels and bridges. Complete with history. 

I had first heard about such plans via BBC News (the exact reporting I found on youtube that I include here). At the time, I thought of it just a political overture. When I read the Jamestown article, China's seems to have been quite serious about the project and even got Taiwan interested at time, albeit maybe more out of curiosity or just for keeping up appearances.

China seems to have planned both bridges and rail tunnels across the strait. According to Jamestown article, it seems to have been the grand project of the current Chinese leader Xi Jinping when he was the party boss of the Fujian province. Xi seems to have taken up the project on national level in the early days of his promotion to the position of China's leader. The project seems to have been placed in freezer after pro-independence Democratic Progressive Party (DPP) came to power in Taiwan in 2016.

A captivating read, in my humble opinion.

https://jamestown.org/program/beijings-ambitions-to-build-cross-strait-transportation-infrastructure/