Underground gas vents
Coming across six or seven of these rusty objects might be unnerving, especially if you find them at a park, the most innocent place you can be. The OP who uploaded this image wanted to know what they were and why they were there. It didn’t take much time for several learned Redditors to provide the answer.
This rusted object is actually an underground gas vent. It turns out that the park was a former landfill, which collects gas and pushes it underground. Several years later, it continues to do so, so these vents allow gasses to flow upwards freely without causing alarm.