Is it plausible to build a tunnel from China to the US through the ocean? Just a wild idea.
China-USA would be a bit longer and go through the ring of fire (https://en.wikipedia.org/wiki/Ring_of_Fire), making it a bigger challenge to build.
I would guess a bare tunnel itself is less of a challenge than its ventilation (even if you build a parallel tunnel tube for ventilation, pumping it out over thousands of kilometers will be challenging. The alternative would be to build ventilation towers every x km) and safety (how long will it take to get an ambulance to a traffic accident? To get people out?)