It was included in the original design of the building by William Thornton and intended to entomb the body of George Washington, the first President of the United States.
[1] When General / President Washington died on December 14, 1799, the Capitol was still under construction and nearing completion of its north wing for the Senate chamber only in its first phase.
The federal government was still then located further north in Philadelphia, Pennsylvania, and wouldn't be moving until about November 1800 and following March 1801, for the opening of the new congressional term and presidential inauguration.
Both houses of Congress later passed a resolution calling for the revered Washington to be entombed as originally planned by the architects in the Capitol upon its eventual construction completion and dedication.
His wife, Martha Washington (1731-1802), agreed to the plan despite the presence in her husband's last will and testament, of a provision he wrote that he was to be buried at Mount Vernon.