One look at the above video and you'd be forgiven for believing you were viewing the inside of someone's elaborate freshwater fish tank. Instead, it's an actual rainforest trail, submerged under millions of gallons of crystalline water.

The unusual scene was captured earlier this month after heavy rains flooded the Rio de la Plata in Brazil. While you would expect such extreme weather to create scenes of frothy chaos and clarity on par with mud, instead the setting at the ecotourism operation Recanto Ecológico Rio da Prata was one of sublime beauty.

"I was attending to a group (of tourists) that day and, instead of walking, we floated," tourism operator Maria Senir Scherer told the Correio do Estado website. "They were certainly privileged because it rarely happens," she said, adding that such flooding has only happened three times over the last 16 years. "The water level is (normally) one meter below the bridge and that day was two meters above it."

Even more remarkable is that this underwater scene was created in only a few hours. Scherer says the heavy rains struck at night, and by dawn the bridge and trail were completely submerged.

So what's going on here? It all comes down to the remarkable geology surrounding Recanto Ecológico Rio da Prata. In addition to its vast array of fauna and flora, it's also home to some of the clearest rivers in the world. This affords surreal snorkeling opportunities, as shown in the video below, to explore an underwater world unlike any other.

The area around these rivers has very porous limestone bedrock which absorbs rainwater, filters it, and then pumps it back out crystal clear into rivers. When the Rio de la Plata flooded, it essentially accelerated this process and "pushed" the clear spring water up and into the surrounding landscape.

" I went to this part of Brazil in November, what happens here is that the water table is high and the land has low spots, so many rivers start there," wrote on commentor on Reddit. "The springs form in mostly flat low areas, so the water had to pool a bit before it starts flowing. You can go snorkeling in all these rivers from there start at a spring, some eventually develop a current and you don't have to pour any effort and just float down the lazy river, others you have to swim to advance down it."

For this particular viral video, the source of all that crystalline water was the spring of the Olho d'água River. You can see the spring in action in the below clip taken in 2016.

Those wishing to experience something similar at the Recanto Ecológico Rio da Prata likely have a better shot at winning the lottery. According to the tour operators, this submerged dreamworld returned to its usual beauty in less than 24 hours.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.