This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.
Q: Between 1631 and 1890, the city tripled its area through land reclamation by filling in marshes, mud flats, and gaps between wharves along the waterfront. The largest reclamation efforts took place during the 19th century; beginning in 1807, the crown of Beacon Hill was used to fill in a 50-acre (20 ha) mill pond that later became the Haymarket Square area. The present-day State House sits atop this lowered Beacon Hill. Reclamation projects in the middle of the century created significant parts of the South End, the West End, the Financial District, and Chinatown. After The Great Boston Fire of 1872, workers used building rubble as landfill along the downtown waterfront. During the mid-to-late 19th century, workers filled almost 600 acres (2.4 km2) of brackish Charles River marshlands west of Boston Common with gravel brought by rail from the hills of Needham Heights. The city annexed the adjacent towns of South Boston (1804), East Boston (1836), Roxbury (1868), Dorchester (including present day Mattapan and a portion of South Boston) (1870), Brighton (including present day Allston) (1874), West Roxbury (including present day Jamaica Plain and Roslindale) (1874), Charlestown (1874), and Hyde Park (1912). Other proposals, for the annexation of Brookline, Cambridge, and Chelsea, were unsuccessful.
A:
What was used to fill a mill pond that became the Haymarket Square?