Apart from being rectangular not triangular, it looks like the settling part of our pig slurry drainage system on the farm where I grew up. Ours had a metal grid over the top to stop folk falling in. It worked by collecting the slurry as it ran down surface drains at the end of each pig pen; the slurry then stood in this tank for a while, then the water part drained on into the pond. The Environmental Protection Agency would have an apoplectic fit to see that set up now. The pond was turned into a horrible slurry pit with a hard crust on top, just thick enough to make you think it would take your weight until you got to about the middle of the pond. Fortunately for me, it was my brother who demonstrated that it wouldn't support a ten yo

Eventually my dad rescued the pond and reinstated it as a haven for wildlife.
Yours could well be working like a small septic tank with the water part draining out into a gravel filled bit below. It should have turned into good well rotted FYM by now, ideal to put on the veggies (or it could stink when you dig it out)