They dominate the skyline nearby Denver Union Station -- several mountains of dirt.The ever-growing piles of fill are reaching higher to the sky and soon will eclipse the height of the iconic structure."These dirt piles when it's completed will be about 40 feet tall and will hold about 70,000 cubic yards of dirt," said Hunter Sydnor of Kiweit Western.The dirt comes from a large swath of land to the west of Union Station. This is the site of the new regional bus facility.Already construction crews have excavated over 100,000 cubic yards of dirt in order to build the underground bus terminal.While the gigantic mounds of dirt are the most visible sign of the construction work on-going at the $145 million project that is slated to open in the spring of 2014, it is what is underground that is the most impressive.Half of the underground bus terminal has been completed. It will allow commuters to switch from bus to light rail while protected from the elements."So when you want to get on a bus you'll go out a door and got on a bus outside there," Sydnor said. "This will be just like an airport concourse. All pretty inside, fancy."Construction crews are working on the eastern part of the underground terminal, where light rail and Amtrak tracks will pass over the subterranean structure."As soon as this half of the bus structure is complete, the second half. It will be back filled with all this dirt," Sydnor said. "And you'll see a lot of work going on inside the bus facility, but also you'll see the commuter rail tracks getting built, the platforms and the really iconic structure that goes over the commuter rail."And the tell-tale giant mounds of dirt will disappear.