The green fields and woods of rural Northamptonshire seem to be the same as the British countryside. There is a thin layer of green pasture next to the farm of Blackgrounds. Beneath the surface lies a cobbled Roman road, walls, wells, pathways and shops, a bustling, prosperous international settlement long lost to memory.

The town is part of the biggest archaeological excavation in British history. There are more than 100 digs and investigations along the 134 miles of the high-speed rail route being built between London and Birmingham. Theenabling works for the much-criticised project.

There are Roman statues of a man, woman and child beneath an abandoned Norman church in the middle of England, as well as a neolithic timber henge aligned to the winter solstice near Wendover.

My tour of Blackgrounds begins in a shipping container filled with laptops, tools and new finds at the site office for Mola, one of the commercial archaeology companies involved in digs for the high speed rail project. Since a law change in 1990 required developers to undertake rigorous archaeological investigations before construction work, a profession once seen as the preserve of the youngest sons of aristocracy has been able to earn its keep outside academia.

Annsofie Witkin with the skeleton of a young woman, shot from above

Catching up with the past: Annsofie Witkin with the skeleton of a young woman. Photograph: Dan Burn-Forti/The Observer

James West, project manager at Blackgrounds, told me that I was standing next to two dead people. There are two buckets filled with bones from centuries ago. West first came to Blackgrounds to dig some test trenches. A Roman villa was found nearby in the 1830s, but was poorly dug and recorded by a typical Victorian excavation. The name of the place indicated that there could be more. Blackgrounds was named after its dark soil because it is mostly pale brown. People created this soil.

There is a really black deposit on top of everything. It's dark because of all the organic material, the wood and bones. I knew there was something special so I fought hard to get back here. An Iron Age settlement that became a prosperous Roman trading town has exceeded my expectations. The scale and quality of finds is better than we thought. It's a bit of a shock to find an undiscovered town in the middle of South Northamptonshire and iron shackles and pottery decorated with hunting scenes.

Without HS2 nobody would have known that this was even here

The scale becomes apparent in the fields as 70 archaeologists pick over the soil while the sound of drilling pierces the still winter air. There are yellow diggers removing top layers, followed by archaeologists with their trowels and wheelbarrows.

British archeology is really good because it is still hand-digging and it gets a lot of people in. His team includes Spanish, Greek, French, German and Australian archeologists.

Most Roman roads are 4-5m wide at a push. To have one 10m wide is quite exciting. Multiple carts could pull up and unload without causing a traffic jam if the road was dual carriageway. The bases of large Roman walls are near the road and show the kind of skilled stone masonry we might associate with a Norman cathedral. West says the town was well planned with walkways, retaining walls, markets and homes built in stone to chest-height with long-vanished wooden upper parts.

Project manager Edward Biddulph.

‘We can see trends in Roman-era population and health’: project manager Edward Biddulph. Photograph: Dan Burn-Forti/The Observer

There are scale weights and bars, as well as rings, necklaces, hair-pins, and pottery from France decorated with a lion chasing a gazelle. These paint a picture, but also create blank canvases.

The team found a lot of nails and coins. Every find is recorded on the gps device. Its location and depth are important data.

Phil Holt of Red River Archaeology is using a metal detector to sweep the soil. Holt laughs and says that his detector is good at depth and discrimination. I will wait until the next layer of soil comes off so I can record the find.

There is a lot of dirt stuck to it so it is hard to see. He likes coins because they can predict a date within a decade. Most of the Blackgrounds are Roman coins that were cheap to make in Britain in the fourth century. Coins made statements. The Emperor dragging a captive with him is depicted in a Valentinian coin. There are a lot of coins. Romans are like that.

Esing a trowel to unearth artefacts.

Deep down: using a trowel to unearth artefacts. Photograph: PR

Archeologists say that without all the earth- moving it creates, these kinds of discoveries wouldn't be made. It was recorded well and saved for future reference.

The area wouldn't have had any work done if the project hadn't been done there. If the land is going to be used for something else, I would like to go in and understand what's there so other people can comprehend it. We are doing something that is going to be looked at for a long time.

Blackgrounds has more to reveal. The road will be removed to look for lost objects. West is excited by the stone well. He says that they will dig down to its bottom. He predicts that there will be more coins, votive offerings and dead animals. He didn't go in willingly.

Puzzles from the past are provided by the dead. Archaeologists for the Consortium of Archaeology companies assembled to meet the demands of the high speed rail project are poring over finds from a medieval village west of Aylesbury. There are two cemeteries with the remains of over 300 people in this undiscovered Roman town. Putlows is a nearby farm that could refer to bodies.

James West in safety gear.

‘I knew there was something special so I lobbied hard to get back here’: James West in safety gear. Photograph: Dan Burn-Forti/The Observer

Annsofie Witkin is examining the skeleton of a woman in her early 20s. Patterns of chronic infections are written in our bones. We can find evidence that he died of a lung infection, but we can't say how he died.

The woman has good teeth, but she was under the age of 19 and under the age of 28. She was 5ft 7in, but had laboured hard from a young age and was showing signs of arthritis in her wrists.

She was beheaded after death in a Roman burial. In Roman Britain, it makes up 2.8% of all inhumations. Witkin doesn't think all of them would be due to crime.

The woman's head was placed over her feet. We don't know if it's hers or not, but we hope the analysis of her genetic material will solve that mystery. The woman's head was cut six times with a fine blade, and it could be that she was not well practised at this. One is very small. Maybe it is a hesitation cut. Maybe it's a family member who has to do this. They are distraught and maybe it was a traumatic miscarriage.

Even after the reports are written, there is more to discover, as the archaeologists point out. Mineral traces within bones can be detected and shown where people were born and raised in Roman Britain.

Below the surface: the well-preserved well at Blackgrounds.

Hidden depths: the perfectly preserved well at Blackgrounds

Archeologists are excited by the big data generated because it allows them to see trends in Roman culture.

Richard Brown has a map on his computer screen. The roads and towns are all Roman. The town of Fleet Marston is at a crossroads of Roman roads.

The significance of the excavations is their sheer quantity. How the archaeology is addressed and where the sites are were prepared for by HS2. It's well planned so we have time to do the job properly and the big data comes from that. We can start drawing conclusions from the large bodies of data.

The finds at Blackgrounds show that Iron Age and Roman cultures are not the same. West says that they have answered the question pretty thoroughly.

The significance of the decapitations and the status of the burials at Fleet Marston is a big question. It is not possible to draw conclusions about early Christianity or not being Christian. It's the same with archeology, it's about a lot of data rather than a single find. It is nice that it is not fixed and known.