Honestly, I wouldn't stress too much about hitting something historic unless you're living smack dab in the middle of an old battlefield or something. Most rain gardens I've done or seen are usually just around 12-18 inches deep—that's generally enough to handle runoff without creating a swampy mess. You don't need to dig a crater in your yard, just enough depth to let water soak into the ground gradually.
But hey, if you're really worried about what's lurking underground (and trust me, I've hit some weird stuff digging around my place), why not grab a metal detector or poke around gently with a garden fork first? Might save you from accidentally unearthing grandma's antique silverware collection or something equally unexpected...
By the way, have you thought about where you'll direct the overflow if there's heavy rain? That's usually the trickier part than the actual digging.
Good points on overflow planning—I learned that lesson the hard way after a heavy storm last spring. Ended up with a mini lake in my flower beds... not ideal. Have you considered adding some gravel or river rocks at the bottom of your rain garden? It helps speed up drainage and prevents standing water issues. Also, if space allows, maybe look into creating a gentle slope or swale to direct excess runoff safely away from your house foundation and toward plants that can handle extra moisture.
Had a similar issue last summer when I drained our above-ground pool. Thought I'd planned it perfectly—hose running out to the back corner of the yard—but didn't realize how quickly that much water could overwhelm the grass. Ended up with a muddy swamp for weeks, and my kids thought it was hilarious (me, not so much...).
What finally worked for me was creating a temporary drainage path using some old bricks and gravel I had lying around. Basically, I built a makeshift channel that gently guided the water toward an area with tougher plants and better drainage. It wasn't fancy or expensive, but it did the trick nicely. If you're on a budget like me, maybe see what materials you already have around the house before buying anything new. Sometimes creativity beats spending money!
"Basically, I built a makeshift channel that gently guided the water toward an area with tougher plants and better drainage."
That's a smart solution. I had to drain our pool for the first time this spring, and I underestimated the water volume too. Ended up with a mini-lake that took forever to dry out. After reading up a bit, I realized spreading the drainage out helps a lot. Instead of one hose, I split the water flow between two hoses, each directed to different corners of the yard. It slowed down the flooding significantly and gave the ground more time to absorb everything. Your idea of using bricks and gravel is great though—wish I'd thought of that earlier. Next time I'll probably combine both approaches to be safe.
Did you notice any erosion issues after directing the water to specific spots? I tried something similar last year, and while it helped with flooding, I ended up with some unexpected soil washout around my shrubs. Wondering if adding gravel or mulch might've prevented that... Your brick and gravel idea sounds promising—might give it a shot next time I drain mine.