Interesting animal videos have been an online staple for years, but lately some seals, whales, and other aquatic mammals have been enjoying some time in the spotlight.

In the past three months, we spotted two heart-warming, adorable clips of seals being released back into the wild, while two videos of a whale rolling and a whale rescue have been viewed over five million times combined.

While those might not be all that unusual, here's a odd trend: recently, we've seen three different, popular videos of aquarium mammals enjoying themselves on the other side of the glass. Yesterday's animal video of the day was this very very strange clip of a Beluga whale enjoying some mariachi music at a Connecticut aquarium. It's been seen nearly half a million times in 24 hours:



Last week, this clip of a baby hippo dancing underwater at the San Diego Zoo picked up roughly the same number of views:



But the whole thing really kicked off back in late May/early July, when footage of this otter -- also from the San Diego Zoo -- playing underwater with a little boy drew over 2 million views when it was picked up by a number of popular blogs and sites all over the world: