Exposed! They tried, but they failed!
Take a look for yourself.
This is the approach to Kingsway, the more modern of the two Mersey Tunnels.
First thing to note is that Google's camera car uses the bit reserved for motorbikes, and goes through the toll plaza without paying.
Then it enters the eastbound tube towards Liverpool, and straight away, it's struggling with the lower light levels. The picture quality quickly deteriorates, and at some point, the camera car appears to jump from the eastbound to the westbound tunnel. Then, the viewer is unceremoniously dumped without warning on the mean streets of Seacombe, without so much as a by-your-leave.
I tried again from other directions, using both tunnels. In each case streetview either jumped to the surface, or just stopped, as if it had hit a dead end.
I have a couple of theories for this.
My first theory is to do with the low light levels. Perhaps some software or human interpreter detected the poor image quality and decided that it was too poor quality to use.
Another theory is that the fact that it goes well below what Google believes to be the level of the land has triggered some kind of error.
Thirdly, Google's card have a complicated camera thing on a pole. I think it's more than just a camera. I think it also contains either GPS, so that it can relate each image precisely to a location. It may even be some kind of uplink. Rather than risk losing data, it is streamed to some remote location via satellite or mobile broadband uplink. Car enters tunnel. Satellite link is broken (satnavs don't work in the tunnel either, although mobile phones do) and the system can no longer function.
I thought I'd found an explanation. But if so wouldn't it be consistent? I mean, if a tunnel is long and deep enough, Street View won't be able to deal with it right?
The Dartford Tunnel is covered by street view, though, and although it's shorter than either of the two Mersey tunnels, it's still long and deep enough (and dark enough) to be stymied by any of my explanations.
So why not the Mersey Tunnels?