A short drive from Indonesia’s cultural capital at Yogyakarta, Borobudur is a wonderfully enchanting temple complex. Built around AD 850, it was already an ancient mystery when the first European explorers arrived, and sunrise on the upper terrace known as the “Sea of Immortality” remains one of the most spiritual experiences I’ve ever had in a lifetime of travel through more than 60 countries.

I was once asked where I’d pick if I could only travel in one country for the rest of my life. Indonesia struck me as the perfect place to cheat such a restriction. “I’d choose Indonesia,” I replied. “Among those 17,508 diverse islands, I could travel for a lifetime without seeing the same thing twice.” Indonesia would surely be a prime contender for the title of “world’s least explored country”. To anyone with a spark of adventurous spirit, it’s thrilling to think that, even today, nobody knows exactly how many islands there are within its vast territory. The 17,508 figure, from an official 1996 “guesstimate”, is widely quoted, but other official estimates range from 13,466 (verified by UN and CIA) to 18,307. The Ministry of Marine Affairs and Fisheries is currently making the first serious official count and we might soon have a real figure.

If you were to island-hop in a direct line along the length of the archipelago from the tip of Sumatra to the border of Papua New Guinea – a distance of almost 6,000km – you would have covered the equivalent of a drive from London to Khartoum or from Los Angeles to Bogota. The sheer scale of this country defies generalisation, but all too often it is summed up in the press simply as “the world’s biggest Muslim nation”. While this is true, it overlooks the incredible cultural diversity of a country where you find entire islands that are predominantly Christian or Hindu or where the major religions are simply overlays for fascinating animist and ancestral religions. It took me a long time to realise that by peering through the spiritual mosquito-netting gauze that was thrown over these communities by missionaries, I would often uncover intriguing ancient beliefs and traditions.

Even among Indonesia’s most mysterious islands, Sumba, near the southern edge of the archipelago, stands out as an enigma. This is a land of legendary warrior tribes – considered more or less untameable by the Dutch colonials – where colourful local cultures still include ritual horse-back spear-battles and megalithic tomb-building reminiscent of the pyramid cultures of the pharaohs. Occasionally, even in the remotest Indonesian wildernesses, you find a world-class hotel like Nihi Sumba Island (regularly voted the world’s best resort), which offers the ultimate in barefoot beach luxury with a chance to experience the colourful culture of an island that stands out as outlandish, even within the perpetually astounding diversity of these islands.

I’ve been lucky to experience much of the variety of wilderness Indonesia: tracking tigers in remote Sumatra and orangutans in Borneo; climbing volcanoes in Sulawesi, Krakatoa and the Maluku Islands; sailing around Sumbawa, Flores and Komodo. The one guaranteed defining factor among all these trips has been the natural hospitality and charm of Indonesians of all ethnic groups and religions. This has been fortunate since only a few islands have world-class luxury accommodation and the only available accommodation has often been a sleeping mat on the floor of a tribal hut or a hammock strung in a jungle clearing.

The Indonesian phinisi has been nominated for UNESCO World Cultural Heritage status, representing as it does an historic ship-building tradition, and these vessels could be vital to the future of tourism here. Around 6,000 Indonesian islands remain uninhabited and these vessels are pioneering routes to islands that have rarely – if ever – seen a visitor. During the northern winter, many of Indonesia’s growing fleet of luxury phinisi cruise Raja Ampat – described by Condé Nast Traveller as “the world’s most beautiful islands” – off the coast of New Guinea Island. But for most of the year they’re based at the port of Labuan Bajo, access point for the Komodo Islands. Crushed between the crumpled volcanic sheets of Sumbawa and Flores, these islands represent a launching pad to worlds you might never have imagined possible.