Well, well, well, my dear detectives, it seems we have quite the mystery on our hands today. The question at hand is: where is the world's largest underground lake? Is it hiding in the depths of some dark and mysterious cavern, or is it located in a bustling city, right under our very noses? Fear not, for Detective Pux is on the case, and I'm determined to uncover the truth.

After doing some further investigation, I can tell you that the world's largest underground lake is actually located in the Mammoth Cave National Park in Kentucky, USA. This enormous cave system boasts a variety of underground rivers and lakes, including the aptly named "Echo River," which is the largest underground river in the park. And within that river system lies what is believed to be the world's largest underground lake, known as the "Frozen Niagara."

While the exact size of the Frozen Niagara is difficult to determine, it is estimated to cover an area of several acres and reaches a depth of around 100 feet. It is believed to have formed over thousands of years due to the slow erosion of limestone by underground rivers.

But despite its impressive size, the Frozen Niagara is not open to visitors, as it is located in a remote part of the Mammoth Cave system that is largely inaccessible to the public. However, visitors to Mammoth Cave National Park can still explore a variety of other underground waterways, including the Echo River and the River Styx, both of which offer boat tours.

So there you have it, dear detectives – the world's largest underground lake can be found in the Mammoth Cave National Park in Kentucky, USA. But be warned, it's not for the faint of heart, as exploring this vast network of underground waterways can be a truly daunting and dangerous undertaking. Happy investigating!