However, the formation of the Seaham Harbour Dock Company and the Londonderry Collieries Ltd by the 6th Marquess swept Seaham into the 20th Century on a tide of industrial and social optimism. Simultaneously, in August 1899, work began on a new South Dock and a new pit at Dawdon.
On 11th November, 1905 the Dock doubled in size and depth, with new quays, staiths, large curving piers and new lighthouse, was opened by Prime Minister Balfour. From that time on the largest ships of the Londonderry fleet like the "Seaham Harbour" and "Maureen" could use the Harbour in perfect safety.
Dawdon Pit developed on the site of the derelict Blast Furnaces. Sinking operations were held up by flooding problems of great magnitude, but a freezing process was successfully adopted and coal production began in 1907. It was boom time in Seaham. Coal exports climbed from 600,000 tons towards 2 million tons in 1911. The Bottleworks, at the height of success, was producing 20 million bottles a year and employing 500 workers. The new Engine Works were experimenting with steam wagons.
Population increased from 10,000 in 1900 to 15,000 in 1911. 900 houses were erected - tenements in the Ropery area and 20 streets of colliery houses at Dawdon. There in 1910, large Council schools for 900 children replaced the tiny Cottages School. Two years later the fine Church of St. Hild and St. Helen (affectionately known by locals as “The Pitmen’s Cathedral”) had been erected, largely by the Londonderry family.
In the central area Presbyterians and Roman Catholics both had striking new churches. Seaham flirted with advanced education by opening the Upper Standard School in 1911.