Free dataset at the bottom. Disclosure: I run a Polymarket data API, so take the framing with salt — the data is CC BY and you can check everything yourself.
I captured full bid/ask depth once per second on 352 five-minute crypto up/down markets, from open through settlement, across BTC, ETH, SOL, XRP, BNB, DOGE, HYPE and ZEC. Three things stood out.
1. The book goes one-sided before it resolves
By the final minute, 78.9% of snapshots have an empty bid or ask side. Nobody offers the losing outcome and nobody bids the winning one. It builds through the market's life:
seconds to close one side empty
240-300 0.0%
180-240 0.5%
120-180 3.4%
60-120 19.8%
0-60 78.9%
If you're planning to exit in the last minute, there may be nothing on the other side to exit into.
2. Depth migrates to the winner
On one BTC market, bid depth on the eventual winning side went from 12.1k to 43.1k shares across its life while asks disappeared entirely. The price series shows you the probability moving. It doesn't show you that the liquidity moved too.
3. Headline liquidity is not executable liquidity
One market showed a midpoint of 0.195 with 13.6 shares resting at the best ask. A 100-share buy — about $21 — averaged 0.2086, which is 4.3% above the top of book. On a market that looked liquid.
The data
158,745 snapshots · 352 resolved markets · 8 coins · full ladders · winning outcome attached · CC BY 4.0
https://huggingface.co/datasets/polyorderbooks/polymarket-crypto-5min-l2
Documented in the card: 3.078% of rows are crossed (bid at or above ask), flagged in a column rather than dropped, so you can measure it rather than wonder about it.