Some anglers caught a strange sight on video while fishing on Lake Vermilion's Anderson Bay during Minnesota's fishing opener. The spectacle, seen in the video below, shows a large area of bubbling or "boiling" water with no apparent cause.

The video and corresponding photos, posted to Facebook by Tom Larson, show that the area appears to be several feet across and appears to be originating from the bottom of the lake. A photo Tom shared in his post shows a sonar image of the phenomenon bubbling up from the bottom, 32 feet below the surface. While it is unknown how long it had been occurring, Tom shared in a comment that had been going on for awhile, and was still going when they left.

The post has drawn a lot of speculation on what could be causing it. While there have been plenty of off-the-wall suggestions like "Russian submarine", "Loch Ness Monster", and someone "pulling the plug on the lake"; others have offered more realistic suggestions. On person commented that it might be a local campground purging water lines, or another person suggested it might be related to a spring on the bottom of the lake.

While the cause is unknown, it is definitely not something you see everyday! Here's the video clip.