Amid the hectic impeachment trial of President Trump, Democratic Rep. Adam Schiff was accused of lying to the American people and denying Trump due process during the investigation.

During the first day of arguments, Trump lawyer Pat Cipollone tore into Schiff and his fellow Democrats, noting that the initial impeachment proceedings took place in secrecy in the “basement of the House” — known as a SCIF, short for Sensitive Compartmented Information Facility.

In his diatribe, Cipollone stated, “The president has been given a minimal due process, nothing here. Not even Mr. Schiff’s Republican colleagues were allowed into the SCIF. Information was selectively leaked out. Witnesses were threatened.”

“By the way, there is still evidence in the SCIF that we have not been allowed to see. I wonder why,” he continued.

So what exactly is a SCIF, and what purpose do they serve? Read on to find out:

What is a SCIF?

A Sensitive Compartmented Information Facility (SCIF) is “a secure room or data center that guards against electronic surveillance and suppresses data leakage of sensitive security and military information,” according to SCIF Global Technologies, a company that builds SCIFs.

These secure rooms can prevent spies from entering undetected and stealing sensitive information.

What are they like?

SCIFs can be just like any other room, according to a 2015 National Journal report on the ones scattered around the Capitol complex.

“It’s just like any other room, but it takes a weight lifter to open the front door,” Rep. Peter King, R-N.Y., said then. “It’s similar to the Situation Room in the White House. It’s austere, but being in there you wouldn’t realize you’re in a SCIF.”

“It’s just like any other room, but it takes a weight lifter to open the front door.” — Rep. Peter King

Before reviewing material in the rooms, people must check their electronic devices with a guard stationed outside and punch in a code, former Rep. Lynn Westmoreland, R-Ga., said.

HOUSE INTEL COMMITTEE TO GET LONG-SOUGHT DOCUMENTS FROM DOJ ON FUSION GPS, NUNES SAYS

The Department of Defense and Director of National Intelligence details the specifications of a SCIF – and the rooms aren’t cheap, National Journal reported. These rooms, which are built based on specific instructions, can cost anywhere from tens of thousands to tens of millions of dollars.

According to the National Journal, these rooms also have to be built by U.S. citizens who work for U.S.-based businesses that are accredited by the Defense Department or another security agency.

The rooms must be built so that passersby cannot hear ongoing conversations and people cannot force their way into them. SCIFs can be a room or even an entire building, National Journal reported.

Fox News’ Jake Gibson contributed to this report.