This is probably Ranunculus sceleratus, fully green in a frozen woodland pond at Potato Creek State Park. In getting this picture, I stepped on a small log in the shallow pond. The log broke, and I went into the icy muck up to my knees. Fortunately the camera and tripod stayed upright on the ice. I was reminded of the time Scott Namestnik and I were botanizing a frozen, degraded wetland near South Bend, Indiana. Scott went through the ice and overtopped his boots, but by the time I got my camera ready, he had climbed out. I tried to talk him into getting back in for a picture, but he wouldn't cooperate!