AN INCREDIBLE time-lapse video reveals how two massive underground tunnels weighing 23 tonnes were squeezed under the railway in Oxford in just four days.

A total of 100 workers contracted by Network Rail worked day and night from July 29 to August 3 to install the flood culverts beneath the tracks at South Hinksey.

The culverts will one day form part of the Environment Agency's Oxford Flood Alleviation Scheme which will protect thousands of city homes and businesses from flooding.

It was an operation the rail firm hadn't even originally planned to undertake.

Network Rail long planned to close the line just south of Oxford station for six weeks in July and August 2016 in order to raise 400m of track by 65cm to reduce the risk of flooding - an £18m project.

But the Environment Agency then asked if it could take advantage of the line closure to get two massive culverts installed under the track as part of its Oxford Flood Alleviation Scheme.

In order to create the 5km flood channel from Botley Road to Sandford-on-Thames to carry flood water away from the city centre, the agency was at some point going to have to go under the railway.

If it had to pay Network Rail to close the line, it would have had to pay millions in compensation, forcing the already steep cost of the scheme up even more.

But, in being able to take advantage of the already-planned line closure, the EA only had to pay Network Rail £1.1m.

And, as this video shows, the rail company's contractors worked against the clock to get the job as quick as possible, though not quite as quick as the video makes it look.