The largest man-made structure on Earth is the Great Wall of China. The second largest is about to be the KM3Net neutrino detector, which will span several cubic kilometers under the Mediterranean, listening for whales. Oh, and looking for neutrinos. That too.
Neutrino detectors are built in the weirdest places, from abandoned mines to icy tunnels deep beneath the south pole. They have to hide out like this to minimize the chance that they'll see anything but neutrinos. The deal with neutrinos, if you remember, is that they're little subatomic particles that aren't electrically charged, meaning that they can pass straight through atoms without even noticing. In fact, 65 billion of them are passing through every square centimeter of your body right this second. PANIC!
The only way we can directly detect neutrinos is when they get unlucky enough to run into the nucleus of an atom, which doesn't happen very often. And by not very often, we mean that your average neutrino would have to pass through a solid brick of metal about a light-year long (!) for it to hit an atom and be detectable. So, even with all of those billions and billions of neutrinos whizzing around you right now, you don't have much to worry about. Neutrino detectors, on the other hand, have a lot to worry about, because neutrinos are so hard to spot. Detectors make up for this by being very, very large, and the new KM3Net is going to be the largest by far.
When it's completed (test detectors are operational now and full-scale construction could start next year), KM3Net will consist of three cubic kilometers of 37,000 individual neutrino sensors under 3,200 feet of water in the Mediterranean Sea. It won't just be able to detect neutrinos, it'll actually be able to track where they came from, which is important since it can clue us in to things like supernovae before* their light actually reaches Earth. So that's all pretty cool, but most importantly, each of K3MNet's detector modules will also include a hydrophone so that we can listen for whale songs at the same time. Now we're talking! SCIENCE!
*We can see neutrino bursts from supernovae before light because neutrinos make it out of a collapsing star itself faster than the energy of the actual supernova does.