If you’re looking to lose something really properly, then dropping it into one of the world’s deepest lakes at night is a good place to start. So, when returning to camp at sunset one day, the irreplaceable brass screw (large, expensive propeller) from a friend’s dhow unwound itself and escaped into the inky depths of Lake Tanganyika, there was never going to be much hope of finding it again. However, the prospect of a morning snorkelling in the crystal clear waters “helping” to look for it was too good an opportunity to forego, no matter how futile an exercise it appeared. So followed 2 sublime hours the next morning. Wearing a mask and snorkel, attached to long ropes, we were gently towed behind a dinghy, up and down the shoreline of this one-mile deep lake. This area of Tanzania is best known for the chimpanzees in the Mahale Mountains and spending time with them is unforgettable, but can be rather like finding yourself stuck on the set of a Marx Brothers film.  So if you’re anything like me, it’ll be the lake that really takes your breath away. The contrast provided by an afternoon drifting peacefully amongst the brightly coloured life of the lake couldn’t be more marked. Nowhere have I experienced such benign water as that of Tanganyika. Clear and sweet as bottled water, perfect in temperature and home to a bewildering array of brightly coloured fish that are an evolutionary parallel to tropical reef fish. Almost all the cichlid fish in Lake Tanganyika are endemic, and many are so pernickety that they’ll only deign to live in the mouth of one particular river on the Lake (although ironically many end up living in England as they’re highly prized as aquarium species). Of course eventually, as midday approached, we had to admit defeat. This prop may have been invaluable, but there's only so much snorkelling you can take and you need a really good reason to miss lunch. So, having trawled many hundreds of metres, we hauled our selves back into the boats and sat on the sides drying in the warm sunshine. And in one of those totally bizarre flashes of serendipity that only Africa seems to be able to deliver, there was the screw. Sitting exactly below us, in clear view on a patch of white sand, not 10 feet below the surface. Lunch that day tasted particularly good, I seem to remember it was fish.