As taken from the author website (www.prestonchild.com):

How to write a sequel that could follow Relic, one of the most successful techno-thrillers ever written? That was the problem facing Douglas Preston and Lincoln Child when they agreed to write Reliquary. What setting could top the New York Museum of Natural History?

That was when Preston and Child realized there was only one solution. And that was to follow the Museum Beast itself. And that way led underground. Deep underground...

Hidden beneath Manhattan is a warren of tunnels, sewers, and galleries, mostly forgotten by those who walk the streets above. There lies the ultimate secret of the Museum Beast...

When two grotesquely deformed skeletons are found deep in the mud off the Manhattan shoreline, museum curator Margo Green is called in to aid the investigation. Margo must once again team up with police lieutenant D'Agosta and FBI agent Pendergast, as well as the brilliant Dr. Frock, to try and solve the puzzle. The trail soon leads deep underground, where they will face the awakening of a slumbering nightmare.