Crates
Bought with $CASH in the book, or dug up by your spade. Every crate is one gear upgrade.

Takes one piece to level 2

Takes one piece to level 3

Takes one piece to level 4

Takes one piece to level 5

Takes one piece to level 6

Takes one piece to level 7

Takes one piece to level 8
Every crate is an upgrade: open it onto a piece of gear (the detector, headphones, spade or ride) and that piece goes up one level. Each crate works on one level, so you climb in order: a Tin Lunchbox takes a piece from level 1 to 2, a Saddlebag from 2 to 3, and so on up to the Time Capsule, which takes it to 8.

All seven crates, each in its own era, opening as the book opens them.
You can only buy crates you can open. The book sells a crate once a piece of your gear is ready for it, and only as many as you have pieces ready. Until then it shows the crate as locked, with what it needs. From a new kit to every piece at level 8 is 28 upgrades.
Your spade can bring a crate up in the hole (see Gear). It is always one your gear can use: the crate for your lowest piece.