These "tunnels" were only at the front of the sheds, an area known to staff as "The Pits."
Going back to the days of the tram, i understand that trams would run into this area, over a series of open pits. This enabled work to be carried out underneath. There were perhaps half a dozen parallel pits, of maybe 60 yards in length, and there were occasional "gaps" connecting these lengthy pits, enabling workers to go from pit to pit, without having to keep climbing in and out. I imagine the tram lines just went over these gaps.
Eventually, as trams were phased out, most of these pits were covered over -rather than filled in. On some of them the front part was left exposed for buses to be worked on. (I remember a bus falling into one of these pits when the shunter got too close to the edge!) In time, probably in the 1980's, these were also covered over, as all maintenance work was done further down the garage.
However, it was still possible to gain entrance to these undercover pits, from the MOT bays further down, and it was possible to walk pretty much the entire matrix of them.
Without prior knowledge of the original use of these pits, they would seem to be a strange, pointless collection of tunnels.
I have one or two photos from the day i went down them, and will see if i can find and post whatever i have. -Don't hold your breath!
There was also at least one air-raid shelter from the war that was also accessable from within the garage.
However, there is nothing left now, as, following the demolition of the old sheds early into this millenium, the entire garage floor and matrix of 'tunnels' were dug up, and filled in, to allow for the new bus parking area, following redevelopment of the depot.
Incidentally, during the preparation works for the new yard, underneath the old bottom garage, the subsoil was NOT found to be merely subsoil. Following research at Birkenhead Library, it transpired that BEFORE becoming tram sheds, at some point in the mid-nineteenth century there was a brick works on the site, and all the clay had been dug up for brick production, and the hole was filled in with all sorts of garbage.
Huge drilling machines had to be brought in, to dig down to the bedrock below, and pilings were inserted to support the new bus parking area!
I may need correcting one or two things in the above, but the general information is correct.