I'm trying to find the name of a sci fi book I read in 1993 about a huge space ship buried under a mountain. It is found one day and the authorities excavate a cave and try to break in but cannot. The cave eventually becomes a tourist attraction and one day a group of visitors somehow do something that opens a portal into the ship, they enter and become trapped inside.
They notice displays of symbols and work out it is a countdown and they have hours or a few days before it tries to take off. They then have to try and find an exit through the huge expanse of the ship. While doing that they learn more and more about the advanced technology and the giant like aliens who built the ship as they navigate their way through the strange vessel.