Holding 66 million gallons of water in one place, the gigantic pool at the San Alfonso Del Mar resort in Chile is the largest in the world. It's so big that the resort allows boats to sail across its 20-acre expanse. This may as well be called a manmade lagoon instead of a swimming pool because it's as long as 20 Olympic-size pools.
Why would a resort need a gigantic pool? Believe it or not, this manmade marvel was a result of a dire need to keep tourists safe. The coastline of Algarrobo, where the resort is located, was home to a large school of jellyfish. In fact, it's not just Algarrobo; much of Chile's long coastline is being plagued by these nefarious stingers every year, due in no small part to climate change.
Pools are built when beaches become too dangerous, at least for the tourism industry. This could also be the case for the homeowner who wants a summer retreat literally close to his home. Pools allow people to take a cool dip without the added risk of marine predators. In the resort's case, the annual maintenance cost of $4 million may be worth it.
Of course, you don't need a pool that big (not that you have enough space for it). For personal enjoyment, your pool could cater to a group of no larger than your entire family tree, or a circle of friends, whichever you prefer.