I love attending Mass at the Cathedral of Saint Matthew the Apostle in Washington, DC. The mosaic behind the altar is breathtaking. I was excited to see they have a 360 tour of the cathedral online here. The online tour also includes explanations of the various artwork, which officially clears up my confusion over the tiny squirrel appearing in the mosaic. It is a reference to Monsignor Thomas Sim Lee (pastor at time of Cathedral construction) and a symbol of steadfastness and hope. Mystery solved.
Take the online tour, or better yet, go in person if you are in DC!