It will be extruded polystyrene insulation wall panels that many people use for intex pool bases. It's very strong and not exactly "soft" but still a touch softer than just ground. I haven't read anywhere yet where something got through this foam.

Here's a nice description of it:
"******* extruded polystyrene insulation is ideal for wall furring, perimeter/foundation, cavity wall, crawlspace, precast concrete, under slab, sheathing and other applications. ******** process technology makes the unique closed-cell structure of ****** insulation highly resistant to moisture, retaining its long-term R-value year after year, even following prolonged exposure to water leakage, humidity, condensation, groundwater and freeze/thaw cycling.

4' x 8' sheet
1" thick
R-5
High R-value (R-5 per inch of product thickness)
Compressive strength of 15 psi
Effective resistance against moisture, corrosion and rot
Lightweight, tough rigid foam panels are easy to handle and install
1" x 4' x 8' extruded polystyrene"

Not sure if I'm allowed to put specific product names out there so I used stars in their place.