It doesn't specify what this particular structure is but this article says about the history of the place

The Army is still struggling with cleaning up Jefferson Proving Ground in southeastern Indiana, part of which became Big Oaks National Wildlife Refuge.

Soldiers test-fired millions of artillery rounds at the proving ground, some made of depleted uranium.

Depleted uranium, a byproduct of nuclear fuel production, is used for armor-piercing shells. Its radiation isn’t strong enough to be dangerous outside the body, but its dust is a serious health risk if inhaled or swallowed, the Environmental Protection Agency says.

Depleted uranium fragments are scattered on the firing range among 1.5 million rounds of unexploded shells, which makes cleanup dangerous and expensive.

The Army told the Nuclear Regulatory Commission it could cost $3.2 billion to clean the area for unrestricted use. Its latest plan calls for waiting 20 years in hopes that better, less expensive technology emerges or the unexploded shells degrade to a safe level.

https://www.latimes.com/world-nation/story/2019-08-18/polluted-us-weapons-sites-are-home-to-wildlife