Even the missionaries failed to make inroads here. Neither Islam nor Christianity has eroded Sumba's ancient Merapu religion and traditional culture. Buffalo, pig and dog are still sacrificed at funerals, and lengths of priceless tie-dyed ikat cloth are buried in massive stone tombs. Little in life or death has changed. Isolation, masking the island like a shroud, has preserved its deepest, darkest rites.

You could hardly call it a busy place, but at certain times of the year there's something that still lures thousands of visitors to Sumba: the Pasola. Held in four different locations in the west of the island a few days after the full moons of February and March, the Pasola is an extraordinary enactment of the island's unique religion. Among Indonesia's many animist festivals, the Pasola ranks head and shoulders above the rest, its danger as intense as its drama.

Literally meaning 'throw spear', the festival centres around a jousting match on horseback between villages representing the rival forces of the Lower and Upper Worlds. Symbolically, this is a battle to keep the balance between between the Merapu, gods of the sky above, and Nyale, the goddess of the watery world below. But in practice the purpose is rather more direct: its main aim is to draw human blood.

If you think this makes the Pasola little more than human sacrifice, you would be right. For human blood is considered the greatest gift that can be given to the gods to keep the balance, and thereby ensure a good harvest. What's more, spectators aren't exempt. When you go Pasola-spotting, you put your life on the line.

On Bali's tame and cosy shores, it sounded irresistible. Who could refuse courting danger and death on an island so strange? I headed east, on the trail of rumours, and could feel Pasola passion mounting as I crossed from shore to shore, edging my way towards the distant Sumba. 'You're going to Pasola?' asked villagers. 'Aduh! Take care!' as they rolled their eyes to heaven and quickly mumbled prayers.

But the closer I got, the more evasive the Pasola became: no-one could tell me exactly when it would begin. Only when I arrived in Sumba itself did I discover that the ratos, high priests, alone know the annual festival date, and even they are dependent onsomething beyond their control: the arrival of a certain multi-coloured seaworm on the beaches of the west coast.

Representing the body of Nyale, who is believed to come to shore to make the land fertile, these squiggly little worms (also named nyale) reveal the Pasola's age-old origins as a harvest and fertility ritual. Significantly, the worms only appear in the first few months of the year, just before the start of the planting season. From their condition, the ratos can tell whether the harvest will be successful: rotten worms indicate a rain-damaged crop; biting, pitted worms a plague of mice. And the colours? Blue worms are said to come from the goddess' hair, yellow ones from her betel-stained lips, and red ones from her blood.

What would happen if the worms this year were bad? Would more human blood have to be spilt? 'Ah, maybe.' The men of the east coast's main town, Wainyapu, had a glint in their eyes when they talked of Pasola. And what of the danger? The spears and the battle? 'The Government has made the fighters use blunted spears now,' they grumbled. 'But injuries still often occur. Sometimes deaths, too. There must always be blood for the gods.' I caught the next bus heading west to Waolabubak, squeezed among men in ikat sarongs and stylish red turbans, daggers tucked in their waistbands. I wasn't a moment too soon, they told me; the Pasola would start the next day. At Waikabubak, trucks were already picking up passengers for the ride to Wanokaka, a small coastal village to the south, and this month's festival site. 'Pasola! Pasola!' yelled those on board, hauling me up into the crush of the crowd.

I expected Wanokaka to be bustling, but instead it was strangely quiet. Only as dusk fell did the atmosphere become charged. From every thatched house came the chatter of voices. Young men appeared on the road, leading horses adorned with bright ribbons. 'What will happen now?' I eagerly asked. But the elders cautioned patience: 'Wait till midnight, then the priests will call from the hilltop.' 'Can I go and see?' They looked at one another and shook their heads. 'It is better to wait.' But I was in no mood to play safe. I found a guide to take me to the priests' sacred hilltop abode, and just before midnight we ventured out. An eerie, tense silence hung over the village. The streets were empty. 'Where is everyone?' I asked the guide. 'They don't like to be out at this time,' he murmured. 'This is when the spirits roam.' He lit a cigarette, and walked a little quicker in the darkness.

Stumbling over the fallen tombs of ancestors in an ancient burial ground, we slowly climbed towards the high priests' house. Suddenly, I heard a sound that stopped me in my tracks: 'Hoo-hoo-hoo-hoooo! Hoo-hoo-hoo-hooo!' Like the howl of wolves, it pierced the night's silence.

'The ratos,' whispered my guide. 'They are calling to the goddess.' And he pointed to a rock where five dark figures stood, daggers at their waists, feathered head-dresses in shadowy outline.

This was no place for uninvited guests. The ratos were angry to see me and I slinked back under the eaves of the house as they called once again to the sea. When the cry came back from priests of the lowland village, I felt caught in the middle. As the ratos scowled at me on their way down the hill, I warily followed, keeping my distance. Low murmurs came from the houses as we passed. No-one was asleep. It was three hours to sunrise and unusually cold.

When we reached the beach where the nyale were due to appear, I found other villagers had already arrived. Other ratos, too - an extraordinary collection of wizened old men in red and black turbans, and ikat sarongs. They stood apart, at the back of the beach, while the rest of us huddled on rocks, watching the sea for a glimmer of light, a ripple of life. But nothing. Only silence and cold.

Gradually, the beach filled with villagers and tourists, and men on horseback holding long, wooden spears. Almost unnoticed, the sky grew pale, the sea a shimmering pink. Suddenly, two ratos approached the water. The crowd got to its feet and surged forward. 'Nyale? Have the nyale come?' The ratos slowly walked into the sea, and bent to scoop up water in their hands. It rippled with colour: the body of the goddess, her hair, her lips, her blood. 'Nyale,' cried the crowd. 'Will the harvest be good?' But even before the old men had pronounced that the worms were fat, the harvest would be fine, the villagers had splashed into the sea, laughing and chattering as they scooped up the worms into baskets and bowls.

'Eat them, they're nice!' cried a jovial old woman, passing me a handful of squirming red worms. 'Raw is good but fried is best,' and she scooped up another batch and flung them in her basket.

The ratos, of course, took no part in these games - they had rituals to perform. They began their own symbolic battle, hurling taunts and curses at each other with astonishing ferocity. Then the oldest rato came forward and crouched on a rock. A dozen chickens were placed before him and one by one he slit their throats, tearing out a handful of feathers from each fowl to add to a pile of betel nuts: ritual offerings for the gods. The chickens were briefly flung onto a fire, their bellies slit and their entrails prodded and inspected.

'Will the harvest be good?' asked the onlookers. 'Yes,' the wise men murmured, 'the harvest will be fine.' But their faces showed no smiles, not a flicker of emotion.

At the other end of the beach, hundreds of horsemen had gathered, bristling with wooden javelins. The first act of the battle was about to begin. Quickly, I joined the front line of spectators, our backs to the sea. The fighters faced each other, their horses ablaze with red ribbons. Suddenly, without warning, they charged, and the air filled with flying spears.

Excitement broke like a wave through the crowd. The strong little Sumba horses, ridden bareback, pounded across the sandy beach. In sallies of ten or 20, the men kept charging at each other, clutching javelins in one hand, throwing them with the other, ducking and dodging an answering hail of spears. The speed and fury was incredible.

Crouched in the sand, I felt the ground throbbing beneath me as the riders suddenly surged towards us. There were no referees, no signals to start or finish an attack, and no barriers to keep the spectators safe from the spears; we, too, were fair game forthe sacrifice. I held my ground and faced the danger - this was what I came for! Within minutes a rider was hit and dragged from the battleground. The crowd cheered. 'Is he dead? Only injured. Blood has been spilt. The gods will be pleased.' After several more sallies, the battle moved inland for the major conflict on a grassy field at the heart of the village. Police took up positions in front of the crowd. I crouched again in the frontline, fired with Pasola passion.

The battleground erupted into a scene of flying spears and charging horses, fiercer than anything we'd seen on the beach. 'Awas! Take care!' gasped my neighbour as horses galloped close by. But almost at once, a volley of spears flew towards us. The crowd ducked and yelled, but two men were hit. My neighbour was one of them.

'Injured!' the crowd yelled, and his assailant triumphantly punched the air, his women supporters screaming a battle cry just as their ancestors had done when their men went hunting for heads.

As the sun rose, the fever of battle grew more intense. The police became edgy, the atmosphere tense. After every short lull, the action picked up again, more violent than ever. Suddenly, a scuffle broke out: riders had abandoned their horses and were throwing rocks at their opponents. Gunfire rang out. In panic, the spectators ran towards the hills, as the police grappled with the horsemen and fired again into the air.

Within seconds it was over: the fight and the Pasola. The horsemen gathered up their spears, and the laughing crowd began to drift away. For the first time in hours, I relaxed. Even the ratos at last were smiling; blood had been spilt, the gods placated, and the harvest ensured.

HOW TO GET THERE Garuda flies daily to Bali. Cost $4730. information supplied by Wallen travel, phone 821-3961