President Donald Trump’s campaign promise of a “big, beautiful” wall is one step closer to happening. The United States Army Corp of Engineers has begun preliminary work on the border wall that is slated to be constructed along the nation’s southern border with Mexico.

According to the The New York Times, the group is “drilling and taking soil samples to determine what type of barriers would be most effective in the different types of geography along the border.” Testing is happening in locations in New Mexico, California and Texas. “The agency has identified the San Diego area and the Rio Grande Valley as priority regions for new border walls,” reads the Times article.

As work moves forward, there are still critical factors necessary for the border wall that are not in place. The design doesn’t appear to be set. Last week, Trump told journalists on Air Force One that the wall could be partially transparent and fitted with solar panels, but no commitments have been made if this will happen. And funding for the wall is not secure. The administration requested $3.6 billion in its 2017 and 2018 budget proposals, but Congress has not committed to providing the money.

A bill approved July 11 by the House Appropriations Committee did include $1.6 billion for the wall—enough to pay for 74 miles of fencing along the southwest border, per The New York Times. In addition, Homeland Security earmarked $20 million that was originally allocated to other programs to fund the construction of four to eight border wall prototypes. The first prototype will be built this summer in San Diego, added to existing border walls.

Opposition continues for construction of the wall, with environmentalists, owners of private property along the intended route and immigrant rights organizations all pushing back.