How about a public park that also serves as a multi-story entrance to Downtown’s extensive underground tunnel system? One that might even provide a little natural light or outdoor seating for below-the-deck diners? This pie-in-the-basement concept for the block-size surface parking lot between One and Two Shell Plaza made an appearance in the Chronicle‘s real estate blog last week — though the architecture firm Gensler had first posted it online this past spring. For the company’s own “Town Square Initiative,” designers were charged with envisioning a new type of town square for various cities around the globe. Tunnel Loop Square, for the block surrounded by Walker, McKinney, Louisiana, and Milam, was one of several proposals stemming from the firm’s Houston office.