By Nigeria correspondent Barnaby Phillips

Deep in the Nigerian rainforest, one of Africa's greatest monuments has been virtually ignored by the outside world for hundreds of years.

Yet the Eredo earthwork lies just one hour's drive from Lagos.

Now being investigated by Dr Patrick Darling of Bournemouth University in the United Kingdom, the Eredo provides clear evidence of a powerful lost kingdom, and,

According to local tradition, the site may even shed light on the legendary Queen of Sheba.

Trekking through the rainforest you could pass within a few yards of the Eredo and not even know it was there - a great ditch hidden in the vegetation.

Built some 1,000 years ago, it encircles the ancient kingdom of Ijebu-Ode, snaking through swamps and forests.

Bigger than pyramids

Dr Darling has walked its length.

"In terms of sheer size it's the largest single monument in Africa - larger than any of the Egyptian pyramids," he says.

The ditch is 160 km (100 miles) long, and in places 20 metres (70 feet) deep.

"Built long before the mechanical era, it was all hand-built, requiring a large labour force and a well co-ordinated labour force working to a master plan," Dr Darling explains.

We make our way through thick tropical vegetation down to the bottom of the Eredo - its smooth walls tower above on either side of us, glowing green with moss.

It is cool and dark, with patches of sunlight filtering through the trees above.

Dr Darling has compiled an immense amount of data on the Eredo, but even he does not know why it was built.

Perhaps to keep elephants out, or as protection against foreign invaders - or perhaps to mark the territorial extent of the Ijebu-Ode kingdom at a time when the rival city states of the Yoruba people were frequently at war with each other.

Sacred place

At a clearing in the forest a local chief, Olaitan Olugbosi, who carries the title of the Baale of Oke-Eiri, is praying by the grave of the woman whom he believes built the Eredo.

Her name is Bilikisu Sungbo - a powerful , childless widow, who wanted a lasting memorial to her name.

Her grave is now a sacred place - thousands of Christians and Muslims come to pray here every year.

As the Baale of Oke-Eiri explains, many of them are convinced that Bilikisu Sungbo is none other than the Queen of Sheba

"All what I'm saying is in the history given to me - when she married King Solomon she became Sheba, Queen Sheba, it's in the Bible like that"

The theory is probably a little far-fetched - the queen of Sheba appears in the biblical Old Testament and in the Koran, leading most historians to believe she lived about 3,000 years ago - a full 2,000 years before the Eredo was built.

Fears for future

But Dr Darling believes that the eredo, threatened by forest clearance and encroaching farmers, needs all the publicity it can get.

He is concerned that in most places, the the side-walls of the trench are giving in and the bottom is eroded because of trees being cut down to create farm land.

"I am worried about it because unless it is properly protected it won't last more than another couple of decades," Dr Darling said.

Even in Nigeria the Eredo is virtually unheard of - yet its existence, and that of similar earthworks in nearby forests, is causing historians to re-think the entire history of West Africa.

It seems that well organised states were being formed in the rainforest region some 500 years earlier than had previously been thought.

But they have left almost nothing behind - except a great ditch through the forest, and a beguiling legend.