The Trump administration’s proposed border wall is expected to be as tall as 30 feet and no shorter than 18 feet to deter potential illegal immigrants, Homeland Security officials said Friday.

U.S. Customs and Border Protection (CBP) has started the process of requesting bids from various companies that would include design proposals and prototypes of President Trump’s proposed barrier along the U.S.-Mexico border, the Washington Times reported.

The agency has released two proposals for contractors who want to submit a bid: a solid, concrete wall and a wall with “a see-through component/capability.”

“The wall design shall be physically imposing in height,” CBP officials said in contracting documents.

The agency said the goal is to have a 30-foot wall, but it will accept proposals as low as 18 feet.

They also said the wall must be at least six feet underground to prevent anyone from tunneling underneath it and high enough that no one can climb the wall without a ladder, NPR reports.

It should take at least an hour, and ideally more than four hours, to make a hole in the wall so agents can stop anyone attempting to break it down.

The government outlines that the wall must be able to withstand a “sledgehammer, car jack, pick axe, chisel, battery operated impact tools, battery operated cutting tools, Oxy/acetylene torch or other similar hand-held tools.”

In the documents, CBP says that the side facing the U.S. must also be “aesthetically pleasing” in “color, anti-climb texture etc., to be consistent with general surrounding environment.” There was no mention of how the Mexican side should look.

About 700 companies have registered as potential vendors for the project since CBP first began weighing interest this month.

Contractors have until March 29 to submit their proposals for the first phase of the project.