There's not a lot of choices for putting hardood below grade. Any solid product is going to give you trouble due to moisture. Only got 2 choices below grade. An engineered floating product. Click lock with a moisture barrier below. Or, a glue down engineered floor. The second choice is going to start with applying an elastomeric coating as a moisture barrier, followed by a flooring adhesive like Bostics Best. The second method is not for the faint of heart. It's a PIA. If you choose the second option, the best advice I can offer is to not spread anymore glue than you can work in a 30 minute period & lay the planks opposite what is considered normal. In other words, work the tonge into the grove rather than the groove into the tongue, otherwise, you're going to plow the grooves full of glue as you pull the planks together.