This is a tough one, and I'm a sucker for time travel stories so I've thought about this a lot, but I chose "Past" because the past is a partly known commodity...but not completely. There's all sorts of stuff we've forgotten. Plenty of it is fine to be forgotten, but the enormous number of details of everyday life were plenty thick in the past. Here's an example:
In the 1920s a neat movie theater/vaudeville house was built in downtown Portland. It had these brass machines in the lobby rigged up with thousands of little signals, to say which seats in the 3,000-seat theater were available so ushers could direct latecomers to seats the most quickly. Decades later, this theater (first known as the Portland, then the Paramount) fell into disrepair and became kind of a hole, where concerts were held, and almost got knocked down. It got remodeled starting in the early '80s to be a symphony hall, the Schnitzer (with the sign outside restored to say "Portland"+). The remodelers dug out these brass devices, cleaned them up, realized what they were, decided to rig them up again -- and couldn't figure out how to do it. 1980s people were flummoxed by 1920s technology. Whoever had known how they worked was long gone.
The past is FULL of little details like that.
+ Actually, that's a slight lie. The sign that had first said "Portland" and then "Paramount" was replaced by a replica. The remodelers misjudged how heavy the original sign was. While they were taking it off the front of the building, the sign tipped over the equipment removing it and the sign went crashing onto the street. It was a Sunday morning in what was then a pretty quiet chunk of downtown Portland, which is a more bustling place now, so no one was hurt, but the original sign was a total loss. Whoops. Luckily, the replacement sign was already being made. And being made lighter.