Dr. Nandkumar M. Kamat

In 2019, I wrote in this column that rainwater hides mysterious microbes. That article came from years of rainwater microscopy in Goa, beginning in 2008, when I started viewing falling rain not as pure water but as a microscopic sample of the atmosphere. The old romantic belief that rainwater falling directly from clouds is the purest form of water has no scientific basis.

I was guided in the detection of biological material in rainwater by an expert in the field, Dr. Brent Christner, now at the University of Florida (https://brent.xner.net/), USA. My work showed that Goan rainwater is a complex atmospheric cocktail containing soluble substances, insoluble particles, microbial forms, dust, aerosols, and other materials transported by monsoon clouds. My PhD student, Dr. Sheela Pal, then confirmed the presence of pathogenic yeast in rainwater directly sampled from the local area. This earlier work led to a larger question. If rainwater can bring down microorganisms, cloud-borne particles, volcanic dust, desert aerosols, and other atmospheric materials, what else is the monsoon delivering silently to Goa?

The answer, seen through years of patient microscopy, is astonishing: Goa’s rainwater carries gold micro-and nanoparticles and auriferous quartz microparticles. This is the result of my continuing rainwater microscopy from 2008 to
the present.

Gold does not fall as visible grains or nuggets. It is present as rare microscopic gold particles in directly sampled fresh rainwater. These particles were detected under a binocular research microscope, with careful scanning and image analysis of each slide made from a single raindrop, which was less than 25 microlitres in volume and took almost an hour to scan. This work is not for the faint-hearted. They exhibit the optical behaviour of gold, including localised surface plasmon resonance (LSPR). Their golden, red-orange, yellow-green, purple, and multicolour responses are characteristic of gold microparticles.

The story goes back to the rainwater work carried out over many years, including the 2013 study at Goa University. At that time, we were already examining monsoon rainwater for unusual microbial and inorganic components. A scientific poster was presented at a national seminar. We investigated whether southwest monsoon clouds could carry unusual biological and mineral signatures. The rainwater slides revealed a world that ordinary water testing would never show. From 2012 onward, gold microparticles were detected in the rainwater. These observations continued through the monsoon seasons until 2019.

Later, when I began working more deeply on secondary gold in Goa, observations of rainwater became even more meaningful. They suggested that gold may not only be present in soils, sediments, and geological settings but may also arrive from the atmosphere in
microscopic form.

This year, the story became even more thrilling. On June 23 and 24, 2026, I sampled fresh rainwater in Mapusa and again obtained positive results. I am still sampling rainwater. For me, every fresh shower now carries a scientific suspense. A slide prepared from falling rain is not simply a slide. This provides a small window into the atmosphere above Goa. The monsoon becomes a living laboratory, and every passing cloud becomes part of an unfinished scientific question; this is not casual microscopy. A slide may take more than an hour to scan properly across multiple optical fields. Without image analysis software and a monitor view, it is easy to miss particles or develop eye strain. The monitor makes it possible to examine the fields patiently and appreciate subtle optical signals. This work demands patience, experience, and knowledge of the optical appearance of gold microparticles. A quick glance through a microscope is insufficient. Therefore, this discovery has
gone unnoticed.

Very few people in Goa or elsewhere examine rainwater in this way, and almost none will use my techniques. Particles appear in many forms. Some are rounded. Some are flake-like. Some of these were irregular fragments. Some appeared as aggregates or clusters of smaller shining particles. Under different optical conditions, they show golden bodies, orange-red centres, yellow-green margins, bright rims, and multicoloured halos. These are not random dust colours but optical behaviours of gold. The same particle may appear subdued in one field and suddenly brilliant in another, depending on its orientation and optical conditions. This changing response is part of the evidence. Gold has a unique relationship with light at very small scales. The surface electrons respond collectively when illuminated, resulting in localised surface plasmon resonance. This is why gold particles can show intense colours and scattering effects depending on their size, shape, surface, and surrounding medium.

In the rainwater images, the LSPR response was among the strongest indicators of these particles’ identity. This is the golden signature of gold itself. The optical response is not an ornamental feature in the images; it is the science speaking
through colour. The visible gold microparticles may only be part of the story. Rainwater may also carry gold nanoparticles (AuNPs), which are far more difficult to detect directly with an optical microscope due to their extremely small size.

In some fields, however, a population of very small particles exhibits LSPR behaviour. This suggests that AuNPs may be present along with larger gold microparticles. If so, the monsoon is not merely bringing down visible microscopic gold grains but also a spectrum of gold particles from the nanoscale to the microscale. This possibility makes rainwater even more fascinating because nanoparticles can travel farther, remain suspended longer, and behave differently from
larger particles.

The monsoon is not only a water cycle. It is also an atmospheric delivery system for dust. The possible origin of this finding is clear. Clouds form around tiny nuclei, such as dust, aerosols, sea salt, mineral particles, microbial matter, and volcanic material. West Asian desert dust is known to travel across the Arabian Sea. Volcanic ejecta and fine mineral aerosols can circulate in the troposphere. Goa, facing the Arabian Sea and receiving intense monsoon rainfall, is well placed to receive such transported materials. If gold-bearing dust, gold nanoparticles, or gold microparticles are present in atmospheric aerosols, the monsoon can bring them down to the surface.

This view is also valid in the broader context of atmospheric transport. The Earth constantly exchanges materials through wind, storms, clouds, and rainfall. Once we accept that the atmosphere is a moving reservoir of particles, the presence of gold microparticles in rainwater is not a fantasy but a logical and testable extension of atmospheric science. This is why I view gold in rainwater as part of a continuous research journey. The first realisation was that rainwater hides microbes and atmospheric life forms. This was followed by evidence of unusual inorganic particles. The next development was the use of gold microparticles.  The line from 2008 to the Goa University work of 2013, the monsoon observations up to 2019, and now to Mapusa in June 2026 is a long scientific story. It is the story of looking again at something familiar until it reveals what it has hidden. This is a pioneering discovery because it requires a different way of looking at rainwater. This requires patience, microscopy, image analysis, and years of repeated observations. Nature keeps its secrets in ordinary places, and science begins when someone decides that the ordinary deserves a closer look.

Goa’s rain has been falling with this secret for two million years now. Under the microscope, the secret shines. In the tiny fields of a wet mount, in the glow of plasmonic colour, and in the patient scanning of fresh rainwater, the monsoon reveals a hidden golden signature.