A visitor to a museum in Portugal was injured last week after he mistook an art installation by British sculptor Anish Kapoor for an optical illusion and fell into a pit.

The incident concerned Kapoor's 1992 installation "Descent into Limbo," which is being shown along with several other works by Kapoor in the Serralves Museum in Porto.

The "Descent into Limbo" has visitors enter a gray building, where they then encounter a 2.5-meter-deep (8-foot-deep) hole. The hole is painted with black pigment, making its depth difficult to perceive.

Thinking the work of art was a black circle painted on the floor, a 60-year-old Italian man attempted to walk near the piece, only to fall several feet into the hole, according to Portuguese newspaper Publico, who first reported on the accident. The man was treated at a nearby hospital for his injuries.

The museum has temporarily closed the exhibit while it works on reinforcing safety measures to reduce risks to visitors. The exhibit already has several signs warning people about the hole as well as a staff member who is present to keep an eye on the visitors.

Kapoor is known for working with perspective-bending materials in his works, which include his 2006 sculpture "Cloud Gate" (also known as "The Bean") in the US city of Chicago.

In 2016, Kapoor purchased the exclusive rights to use Vantablack, a substance created by British researchers, in artistic works. Vantablack is a series of nanotubes that reflect little to no light, making it one of the darkest substances on Earth.

It was unclear whether Vantablack was used in the "Descent into Limbo" installation.