First tunnel under a navigable waterway

The first tunnel to be completed beneath a navigable waterway is the 365-m-long (1,300ft) Thames Tunnel, which spans the Thames between Rotherhithe and Wapping in London, UK. It was designed by French-British engineer Marc Brunel with the assistance of his son Isambard Kingdom Brunel and funded by the government-backed Thames Tunnel Company. Construction began on 2 March 1825 and after many delays, accidents and near-disasters, the tunnel was officially opened to the public on 25 March 1843.

In the early 19th century, London was a hub of international trade and industry. Its population had more than doubled in the preceding 50 years and its ports and dockyards, which lined both banks of the Thames, were the busiest in the world.

This city, soon the be the largest in the world, was divided by the tidal estuary of the Thames, and there were only a handful of bridges for foot and wagon traffic. Moreover, there were no crossings over the stretch of river known as the Pool of London, which extended downstream from London Bridge and served as the British Empire's most important port.

In the age of tall-masted sailing ships, a bridge over this stretch of the river was an impossibility, but a crossing was clearly needed to handle the volume of goods and people that constantly needed to cross.

Several engineers and businessmen – most with backgrounds in the mining industry – had proposed the construction of a tunnel, but these projects had all run afoul of financial or technical problems. The most significant of these was the Thames Archway Company, who employed Richard Trevithick (inventor of the high-pressure steam engine) to oversee the construction of a tunnel in 1807.

When this tunnel flooded in 1808, investors abandoned the plan. It was revived, however, in the early 1820s when the French-born engineer Marc Brunel – who had patented a device called a "tunneling shield" in 1818 – draw up plans for a tunnel that could connect the banks of the Thames between Rotherhithe and Wapping.

Construction began with the sinking of the access shafts in March 1825, and work started on the actual horizontal tunnel towards the end of the year. Key to the success of the project was the tunneling shield developed by Brunel. This was a cast iron structure, around 7 m high and 12 m wide, but only a 2–3 metres deep. It was clad in iron plates on the top, bottom and sides, but open at the front and back. It had three floors inside, divided up into 12 "cells" each of which was manned by a laborer. As the workers cut away the material in front of the shield, it was slowly inched forward on jacks while bricklayers sealed the walls behind it.

The construction was dangerous, but perhaps not for the reasons one would expect. One of the most significant risks to the workers was the filthy, sewage-laden water seeping down from the river above, which made many people sick. The tunnel flooded four times during construction – the second time almost killing Marc Brunel's son, Isambard – but it was repaired and drained both times.

Progress continued, slowly but surely, for 18 years. The main impediment to the project was a perpetual shortage of money, as building the tunnel had turned out to be a far longer and more expensive undertaking than Brunel had anticipated.

It was eventually completed in March 1843, and opened in a grand ceremony led by the Duke of Wellington. The expected profits never rolled in for the Thames Tunnel Company, however. The tunnel's awkward approaches made it unsuitable for most goods traffic, and its entrances' locations in poor industrial areas (neighborhoods described by one commentator as "terra incognita to the civilized portion of mankind") put off well-heeled visitors.

The tunnel had cost £634,000 to build (equal to about £67 million in 2023), but for the next 20 years it was little used for anything other than a novelty market. The company was eventually rescued by the development of what would become the London Underground. The East London Railway Company purchased the tunnel for use as a railway crossing in 1865 for £800,000 (equal to £83 million in 2023). Today the tunnel is still used as part of the confusingly named London Overground, a metro line operated by Transport for London.

Although the Thames Tunnel is the first that survives, there is some evidence to suggest that there may have been another tunnel built around 4,000 years earlier beneath the Euphrates River in Babylon (an ancient city located around 80 km south of present-day Baghdad, Iraq). Several classical historians, including Diodorus (fl. 50 BCE) and Philostratus (fl.250 CE) mention a cut-and-cover tunnel, constructed of stone and waterproofed with bitumen, that was commissioned by Queen Semiramis around 2160 BCE.

It should be noted, however, that no archaeological evidence of this tunnel has ever been found, and that the surviving accounts of its construction are as distant in time from the events they describe as we are from the Roman Republic. Moreover, the construction techniques described by these historians (cut-and-cover tunelling, barrel-vaulted brickwork) bear more of a resemblance to the engineering practices of the Roman Empire than to anything the Babylonians are known to have done.