Imponderabilia (1977) was a collaborative performance by  artist Marina Abramovic and Ulay, in which they flanked the entrance to the Galleria Communale d’Arte Moderna in Bologna, Italy—completely nude.

Visitors entering the museum were forced to squeeze in between their bodies. Interestingly, we see that many of the visitors avoided eye contact or facing either person. The performance raises questions about  personal boundaries and spatial limits.